MERCK & CO., INC., 11-K filed on 6/24/2026
Annual Report of Employee Stock Plans
v3.26.1
Cover
 12 Months Ended
Dec. 31, 2025
Document Information [Line Items]  
Document Type 11-K
Entity Registrant Name MERCK & CO., INC.
Entity Central Index Key 0000310158
Amendment Flag false
EBP 061  
Document Information [Line Items]  
Document Type 11-K
Entity tax identification number 98-0658260
Plan number 061
Entity Registrant Name MERCK & CO., INC.
Entity Central Index Key 0000310158
Amendment Flag false
v3.26.1
Statements of Net Assets Available for Benefits - EBP 061 - USD ($)
$ in Thousands
Dec. 31, 2025
Dec. 31, 2024
    Investments, at fair value    
        Investment in Master Trust $ 244,537 $ 228,777
        Investment in Master Trust, at fair value 73 43
            Total investments 244,610 228,820
    Receivables    
        Employer contributions 12 27
        Participant contributions 36 67
        Notes receivable from participants 2,267 2,688
            Total receivables 2,315 2,782
Net assets available for benefits $ 246,925 $ 231,602
v3.26.1
Statement of Changes in Net Assets Available for Benefits - EBP 061
$ in Thousands
 12 Months Ended
Dec. 31, 2025
USD ($)
    Investment gain  
        Plan interest in Master Trust investment gain $ 33,756
    Interest income on notes receivable from participants 207
    Contributions to the Plan  
        By participants 4,826
        By employer 1,754
            Total contributions 6,580
            Total additions 40,543
Deductions from net assets attributed to  
Participant withdrawals (25,211)
Administrative expenses (9)
            Total deductions (25,220)
            Net increase 15,323
Net assets available for benefits  
    Beginning of year 231,602
    End of year $ 246,925
v3.26.1
Description of Plan
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Description of Plan [Line Items]  
Description of Plan Description of Plan
The following description of the MSD Puerto Rico Savings & Security Plan (the “Plan”) provides only general information. Participants should refer to the Plan document for a more complete description of the Plan’s provisions.
General
The Plan is a defined contribution plan established effective July 1, 1997. The Plan is a profit sharing plan designed to provide a systematic means of saving and investing for the future and an opportunity for employees of MSD International GmbH (Puerto Rico Branch) LLC and the Puerto Rico branch of Merck Sharp & Dohme (I.A.) LLC (the “Companies”) to become shareholders of Merck & Co., Inc. (“Merck”). Regular full-time employees of the Companies, as defined in the Plan document, are eligible to enroll in the Plan as soon as administratively feasible after such employee begins service with one of the Companies. MSD International GmbH (Puerto Rico Branch) LLC, a branch of MSD International GmbH, a Swiss limited liability company, is the Plan sponsor (the “Sponsor”). The Plan’s recordkeeper is Fidelity Investments Institutional Services Company (“Fidelity” or the “Recordkeeper”). Banco Popular de Puerto Rico serves as the Plan’s trustee (the “Trustee”).
Participants direct the investment of their contributions into any fund investment option available under the Plan, including Merck common stock. The Plan offers a three-tiered approach to investing:
*Retirement Portfolios - a custom set of target date retirement funds;
*Core Funds - a selection of investment options (active and passive) that span a range of asset classes;
*Fidelity Brokerage Link® - a self-directed brokerage account that provides access to more than 8,500 mutual funds.
The Plan is administered by management committees appointed by the Sponsor’s Assistant Vice President - Plant Management, the Compensation and Management Development Committee of the Board of Directors of Merck or their delegates.
The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”).
Master Trust
A portion of the assets of the Plan are maintained, for investment purposes only, with all the assets of the MSD Employee Stock Purchase and Savings Plan, and the Merck US Savings Plan in the Merck & Co., Inc. Qualified Plans Master Trust (the "Master Trust)". The plans own specific interests in the Master Trust assets.
The portion of Master Trust assets allocable to each plan is based upon the participants’ account balances within each plan. Investment income for the Master Trust is allocated to each plan based on the specific investments held in the Master Trust.
Contributions
Participants may contribute up to 25% of their annual eligible compensation on a before-tax basis. However, before-tax contributions cannot exceed the lesser of: (i) the amount as in effect under Section 402(g) of the U.S. Internal Revenue Code of 1986, as amended (the “U.S. Code”) or Section 1081.01(d)(7)(A) of the Internal Revenue Code for a New Puerto Rico of 2011, as amended (the “2011 PR Code"); or (ii) in the case of highly compensated employees, the average actual deferral percentage limit. Also, total annual additions to a participant's accounts during a plan year are limited to the lesser of: (i) the applicable limit for a particular tax year under Section 415(c) of the U.S. Code as adjusted by the Secretary of the Treasury; or (ii) one hundred percent (100%) of the participant's compensation paid by the employer during the calendar year. Participants may also contribute up to 25% of their annual eligible compensation on an after-tax basis subject to the limit provided under Section 1081.01(a)(15). Upon reaching the statutory elective deferral limit, or upon reasonable expectations that any of the aforementioned limits will be exceeded, the participant may elect to suspend contributions for the remainder of the plan year or elect to continue on an after-tax basis.
In addition, the Companies make matching contributions of $0.75 for every $1.00 of an employee's contributions, up to 6% of such employee's eligible compensation (maximum match is 4.5% of eligible compensation) per pay period (to the statutory limit). The combination of before-tax contributions, after-tax contributions and Companies matching contributions may not
exceed the statutory limit ($70,000 in 2025). Participant and Companies matching contributions are invested according to a participant’s elections.
Participants are automatically enrolled in the Plan 60 days after first becoming eligible with a before-tax base pay contribution rate of 6%. In addition, each participant with a before-tax base pay contribution rate of at least 1% may elect to participate in an annual automatic increase program, in whole percentages of at least 1% and not more than 3% per year. The automatic percentage increase may not cause a participant’s before-tax base pay contribution rate to exceed 25%.
In addition, the Plan permits unmatched before-tax “catch-up contributions” of up to $1,500 by participants who are at least age 50 by year-end.
Participant Accounts
Each participant’s account is credited with the participant’s contributions, the Companies’ contributions, and an allocation of Plan earnings. The allocation is based on participants’ account balances, as defined in the Plan document.
Vesting
Participants are immediately vested in their contributions, all Companies’ matching contributions, plus actual earnings thereon.
Notes Receivable from Participants
Participants may borrow from their account balances with interest charged at the prime rate in effect on the last business day of the prior month plus 1%. Once set, the interest rate is in effect for the duration of the loan. Loan terms range from one to five years for a short-term loan or up to fifteen years for the purchase of a primary residence and bear interest at rates that range from 4.25% to 9.50%. The minimum loan is $1,000 and the maximum loan is the lesser of (i) $50,000 less the highest outstanding loan balance(s) during the one year period prior to the new loan application date, or (ii) 50% of the participant’s account balance less any current outstanding loan balance(s) and defaulted loan amounts. Principal and interest are paid ratably through payroll deductions. Loan terms are grandfathered into the Plan in accordance with the legacy plan guidelines, including loan terms for participants that transferred into the Plan from a legacy plan.
Payment of Benefits
Participants are entitled to receive automatic, voluntary, in-service (which include hardship withdrawals), or mandatory distributions as provided in the applicable Plan provisions.
v3.26.1
Summary of Accounting Policies
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Accounting Policy [Line Items]  
Summary of Accounting Policies Summary of Accounting Policies
Basis of Accounting
The accompanying financial statements are prepared on the accrual basis of accounting.
Use of Estimates
The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and changes therein, and disclosure of contingent assets and liabilities at the date of the financial statements. Management believes that these estimates are adequate. Actual results could differ from those estimates.
Investment Valuation and Income Recognition
All investments are recorded at fair value in the accompanying financial statements. Valuation of investments of the Plan that are in the Master Trust represents the Plan’s specific interest in the Master Trust. The Plan’s investment in the Master Trust is stated at fair value and is based on the beginning of year value of the Plan’s interest in the Master Trust plus actual Plan contributions and specific investment income less Plan distributions, specific investment losses and allocated expenses.
Purchases and sales of securities are recorded on a trade-date basis. Dividend income is recorded on the ex-dividend date. Interest income is recorded as earned. Realized gains and losses from security transactions are reported on the average cost method.
Notes Receivable from Participants
Notes receivable from participants are measured at their unpaid principal balance plus any accrued but unpaid interest. Delinquent participant loans are reclassified as benefit payments based upon the terms of the Plan document. No allowance for credit losses has been recorded as of December 31, 2025 and 2024.
Contributions
Employee and matching contributions by the Companies are recorded in the year in which participant compensation is earned.
Payment of Benefits
Benefits are recorded when paid.
Administrative Expenses

Administrative expenses primarily include loan set up fees. Other administrative expenses are paid by the Companies.
Risks and Uncertainties
The Plan provides for various investment options in investment securities. Investment securities, in general, are exposed to various risks and may decline in value for a number of reasons, including changes in prevailing interest rates and credit availability, increases in defaults, increases in voluntary prepayments for investments that are subject to prepayment risk under normal market conditions, widening of credit spreads, and overall market volatility. Due to the level of risk associated with certain investment securities, it is reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participants’ account balances and the amounts reported in the Statements of Net Assets Available for Benefits.
v3.26.1
Related-Party Transactions
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Related Party and Party-in-Interest Transactions [Line Items]  
Related-Party Transactions Related-Party Transactions
Certain Plan investments are shares of registered investment companies (mutual funds) and common/collective trusts managed by the Recordkeeper or its affiliates. Fidelity is the Recordkeeper as defined by the Plan and, therefore, these transactions qualify as party-in-interest transactions. The total market value of the Plan’s portion of the investments managed by the Recordkeeper was $8.5 million and $7.0 million at December 31, 2025 and 2024, respectively. During 2025, the Plan’s portion of interest and dividends, realized and unrealized gains from investments managed by the Recordkeeper was $0.04 million, $0.35 million and $2.67 million, respectively.
Merck also is a party-in-interest to the Plan under the definition provided in Section 3(14) of ERISA. Therefore, Plan transactions of Merck common stock qualify as both related party transactions and party-in-interest transactions. The market value of the Plan’s portion of the investment in Merck common stock was $29.3 million and $29.5 million at December 31, 2025 and 2024, respectively. During 2025, the Plan’s portion of dividends, realized gains and unrealized gains from Merck common stock was $0.9 million, $1.4 million and $0.1 million, respectively. The Plan’s portion of purchases and sales of Merck common stock during 2025 was $1.0 million and $2.7 million, respectively.
v3.26.1
Plan Termination
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Description of Plan [Line Items]  
Plan Termination Plan Termination Although they have not expressed any intent to do so, the Companies have the right under the Plan to discontinue their contributions at any time and to terminate the Plan subject to the provisions of ERISA. Upon termination of the Plan, each participant thereby affected would receive the entire value of his or her account as though he or she had retired as of the date of such termination.
v3.26.1
Tax Status
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Tax Status [Line Items]  
Tax Status Tax Status
The Plan is qualified under Section 1081.01 of the 2011 PR Code and with Section 401(a) of the U.S. Code, and has received a favorable determination letter from the Puerto Rico Department of Treasury (the "PR Treasury") dated November 12, 2015, and from the U.S. Internal Revenue Service dated October 6, 2015, as to its qualified status under the 2011 PR Code and the U.S. Code, respectively. The Trust associated with the Plan is intended to be exempt from Puerto Rico income taxation pursuant to the provisions of Section 1081.01(a) of the 2011 PR Code. The Plan's Master Trust is to be considered as an organization as described in Section 401(a) and exempt under Section 501(a) of the U.S. Code. The Plan has been amended since receiving the determination letters. However, the Sponsor believes that the Plan is designed and currently operates in compliance with the 2011 PR Code and U.S. Code. Accordingly, no provision for income taxes has been made. Finally, the Trust has complied with the requirements established by Section 1022(i)(1) of ERISA.

Accounting principles generally accepted in the United States of America require Plan management to evaluate tax positions taken by the Plan and recognize a tax liability (or asset) if the Plan has taken an uncertain position that more likely than not would not be sustained upon examination by the PR Treasury pursuant to the provisions of the 2011 PR Code. The Companies have analyzed the tax positions taken by the Plan and have concluded that, as of December 31, 2025 and 2024, there are no uncertain tax positions taken or expected to be taken that would require recognition of a liability (or asset) or disclosure in the financial statements. The Plan is subject to routine audits by taxing jurisdictions. The Plan administrator believes the Plan is no longer subject to income tax examinations for years prior to 2021.
v3.26.1
Master Trust
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Master Trust [Line Items]  
Master Trust Master Trust
The net assets of the Master Trust and the Plan's interest are as follows:

2025
(in thousands)Master TrustPuerto Rico
Registered investment companies (mutual funds)$97,747 $1,674 
Common/collective trusts12,677,091 171,262 
Equity securities2,307,397 33,498 
Fixed income securities406,994 7,217 
Self-directed brokerage accounts459,800 1,048 
Merck common stock907,977 29,306 
Accrued interest and dividends15,877 389 
Other net assets8,648 143 
$16,881,531 $244,537 
2024
(in thousands)Master TrustPuerto Rico
Registered investment companies (mutual funds)$500,106 $7,602 
Common/collective trusts11,107,463 151,225 
Equity securities2,244,340 32,895 
Fixed income securities386,182 6,655 
Self-directed brokerage accounts368,784 431 
Merck common stock1,026,831 29,511 
Accrued interest and dividends17,180 388 
Other net assets4,994 70 
$15,655,880$228,777
Total investment income (loss) of the Master Trust for the year ended December 31, 2025, is as follows:
2025
(in thousands)Master Trust
Investment income
    Net appreciation$2,354,281 
    Interest and dividends157,490 
        Total investment income$2,511,771 
v3.26.1
Fair Value Measurements
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Investment, Fair Value and NAV [Line Items]  
Fair Value Measurements Fair Value Measurements
Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Entities are required to use a fair value hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs when measuring fair value. There are three levels of inputs that may be used to measure fair value:
Level 1 – Quoted prices in active markets for identical assets or liabilities. The Master Trust's Level 1 investments primarily include registered investment companies (mutual funds), self-directed brokerage accounts and common stocks.
Level 2 – Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. The Master Trust's Level 2 investments primarily include investments in certain fixed income investments such as government and agency obligations, corporate obligations and mortgage and asset-backed securities.
Level 3 – Unobservable inputs that are supported by little or no market activity and that are financial instruments whose values are determined using pricing models, discounted cash flow methodologies, or similar techniques, as well as instruments for which the determination of fair value requires significant judgment or estimation. The Master Trust did not hold any Level 3 investments at December 31, 2025 and 2024.
If the inputs used to measure the financial assets and liabilities fall within more than one level described above, the categorization is based on the lowest level input that is significant to the fair value measurement of the instrument. There have been no changes in the valuation methodologies used at December 31, 2025 and 2024.
Within the Plan and Master Trust, investments are recorded at fair value as follows:
Registered Investment Companies (Mutual Funds)
Registered investment companies (mutual funds) are valued at their respective net asset values. The net asset values are typically determined by the fund at the close of regular trading. Investments in registered investment companies (mutual funds) generally may be redeemed daily.
Common/Collective Trusts
The common/collective trusts are valued at their respective net asset values, as a practical expedient for fair value. The fair value of investments in the common/collective trusts are determined by their trustee. The Master Trust's investments in common/collective trusts generally may be redeemed daily.
Fixed Income Securities
Fixed income securities, including U.S. government and agency obligations, corporate obligations and mortgage and asset-backed securities, are generally valued on the basis of valuations furnished by a pricing service approved by the custodian or at fair value as determined in good faith by the custodian and the Companies. The custodian has the discretionary authority to hire a pricing service to determine valuations using methods based on current market transactions, prices for comparable securities and various relationships between securities which are generally recognized by institutional traders.
Self-Directed Brokerage Accounts
Self-directed brokerage accounts are comprised of mutual funds, which are valued at the net asset value reported by the managers of the funds, typically determined at the close of regular trading.
Equity Securities / Common Stocks
Common stocks, for which market quotations are readily available, are generally valued at the last reported sales price on their principal exchange on the valuation date, or official close price for certain markets. If no sales are reported for that day, these investments are valued at the more recent of (i) the last published sale price or (ii) the mean between the last reported bid and asked prices for long positions, or at fair value as determined in good faith by the Trustee and the Companies.
Investments Measured at Fair Value

Investments at fair value as of December 31, 2025 are summarized below:

December 31, 2025
Fair Value Measurements Using
(in thousands)Quoted PricesTotal
In ActiveSignificant
Markets forOtherSignificant
Identical ObservableUnobservable
AssetsInputsInputs
(Level 1)(Level 2)(Level 3)
Interest bearing cash$73 $— $— $73 
Total assets$73 $— $— $73 
Investments in the Master Trust
Registered investment companies$97,747 $— $— $97,747 
Equity securities2,307,397 — — 2,307,397 
Fixed income securities— 406,994 — 406,994 
Self-directed brokerage accounts459,800 — — 459,800 
Merck common stock907,977 — — 907,977 
Investments measured within the fair value hierarchy$3,772,921 $406,994$— $4,179,915 
Common/collective trusts measured at NAV practical expedient (1)
$12,677,091 
Master trust investments at fair value$16,857,006 

(1) Certain investments that were measured at net asset value (NAV) per share or its equivalent have not been classified in the fair value hierarchy. The fair value amounts presented in this table also do not include the Master Trust's accrued interest and dividends or other net assets.

The Plan's interests within the Master Trust as specifically identified in Note 6 are leveled in the same manner as the Master Trust investment categories presented above.
Investments Measured at Fair Value

Investments at fair value as of December 31, 2024 are summarized below:
December 31, 2024
Fair Value Measurements Using
(in thousands)Quoted PricesTotal
In ActiveSignificant
Markets forOtherSignificant
Identical ObservableUnobservable
AssetsInputsInputs
(Level 1)(Level 2)(Level 3)
Interest bearing cash$43 $— $— $43 
Total assets$43 $— $— $43 
Investments in the Master Trust
Registered investment companies$500,106 $— $— $500,106 
Equity securities2,244,340 — — 2,244,340 
Fixed income securities— 386,182 — 386,182 
Self-directed brokerage accounts368,784 — — 368,784 
Merck common stock1,026,831 — — 1,026,831 
Investments measured within the fair value hierarchy$4,140,061 $386,182 $— $4,526,243 
Common/collective trusts measured at NAV practical expedient (1)
$11,107,463 
Master trust investments at fair value$15,633,706 

(1) Certain investments that were measured at net asset value (NAV) per share or its equivalent have not been classified in the fair value hierarchy. The fair value amounts presented in this table also do not include the Master Trust's accrued interest and dividends or other net assets.

The Plan's interests within the Master Trust as specifically identified in Note 6 are leveled in the same manner as the Master Trust investment categories presented above.
v3.26.1
Subsequent Events
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Subsequent Event [Line Items]  
Subsequent Events Subsequent EventsThe Plan has evaluated subsequent events through the date the financial statements were issued and no events were noted which warrant adjustments to, or disclosure in the financial statements.
v3.26.1
Schedule H, Line 4i – Schedule of Assets (Held At End of Year)
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Schedule of Asset Held for Investment [Line Items]  
Schedule H, Line 4i – Schedule of Assets (Held At End of Year)
MSD Puerto Rico Savings & Security PlanSchedule H
Schedule H, Line 4i - Schedule of Assets (Held at End of Year)
December 31, 2025
(a)(b)(c)(d)   (e)
Identity of Issue, Borrower, Lessor or Similar PartyDescription of Investment Including Maturity Date, Rate of Interest, Collateral, Par or Maturity ValueCostCurrent Value
**Master TrustInvestment in Master Trust***$244,537,069 
*  Banco Popular  Interest bearing cash account***73,226 
*  Notes receivable from participants
Interest rates ranging from 4.25% to 9.50% and with maturities through 2038
2,267,542 
Total$246,877,837 
*Denotes a party-in-interest to the Plan.
**There are certain investments within the Master Trust that are party-in-interest.
***Cost information not required to be presented for participant directed investments.
v3.26.1
Summary of Accounting Policies (Policies) - EBP 061
 12 Months Ended
Dec. 31, 2025
EBP, Accounting Policy [Line Items]  
Basis of Accounting
Basis of Accounting
The accompanying financial statements are prepared on the accrual basis of accounting.
Use of Estimates
Use of Estimates
The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and changes therein, and disclosure of contingent assets and liabilities at the date of the financial statements. Management believes that these estimates are adequate. Actual results could differ from those estimates.
Investment Valuation and Income Recognition
Investment Valuation and Income Recognition
All investments are recorded at fair value in the accompanying financial statements. Valuation of investments of the Plan that are in the Master Trust represents the Plan’s specific interest in the Master Trust. The Plan’s investment in the Master Trust is stated at fair value and is based on the beginning of year value of the Plan’s interest in the Master Trust plus actual Plan contributions and specific investment income less Plan distributions, specific investment losses and allocated expenses.
Purchases and sales of securities are recorded on a trade-date basis. Dividend income is recorded on the ex-dividend date. Interest income is recorded as earned. Realized gains and losses from security transactions are reported on the average cost method.
Notes Receivable from Participants
Notes Receivable from Participants
Notes receivable from participants are measured at their unpaid principal balance plus any accrued but unpaid interest. Delinquent participant loans are reclassified as benefit payments based upon the terms of the Plan document. No allowance for credit losses has been recorded as of December 31, 2025 and 2024.
Contributions
Contributions
Employee and matching contributions by the Companies are recorded in the year in which participant compensation is earned.
Payment of Benefits
Payment of Benefits
Benefits are recorded when paid.
Administrative Expenses
Administrative Expenses

Administrative expenses primarily include loan set up fees. Other administrative expenses are paid by the Companies.
Risks and Uncertainties
Risks and Uncertainties
The Plan provides for various investment options in investment securities. Investment securities, in general, are exposed to various risks and may decline in value for a number of reasons, including changes in prevailing interest rates and credit availability, increases in defaults, increases in voluntary prepayments for investments that are subject to prepayment risk under normal market conditions, widening of credit spreads, and overall market volatility. Due to the level of risk associated with certain investment securities, it is reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participants’ account balances and the amounts reported in the Statements of Net Assets Available for Benefits.
v3.26.1
Master Trust (Tables)
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Master Trust [Line Items]  
Schedule of master trust net assets
The net assets of the Master Trust and the Plan's interest are as follows:

2025
(in thousands)Master TrustPuerto Rico
Registered investment companies (mutual funds)$97,747 $1,674 
Common/collective trusts12,677,091 171,262 
Equity securities2,307,397 33,498 
Fixed income securities406,994 7,217 
Self-directed brokerage accounts459,800 1,048 
Merck common stock907,977 29,306 
Accrued interest and dividends15,877 389 
Other net assets8,648 143 
$16,881,531 $244,537 
2024
(in thousands)Master TrustPuerto Rico
Registered investment companies (mutual funds)$500,106 $7,602 
Common/collective trusts11,107,463 151,225 
Equity securities2,244,340 32,895 
Fixed income securities386,182 6,655 
Self-directed brokerage accounts368,784 431 
Merck common stock1,026,831 29,511 
Accrued interest and dividends17,180 388 
Other net assets4,994 70 
$15,655,880$228,777
Total investment income (loss) of the Master Trust for the year ended December 31, 2025, is as follows:
2025
(in thousands)Master Trust
Investment income
    Net appreciation$2,354,281 
    Interest and dividends157,490 
        Total investment income$2,511,771 
v3.26.1
Fair Value Measurements (Tables)
 12 Months Ended
Dec. 31, 2025
EBP 061  
EBP, Investment, Fair Value and NAV [Line Items]  
Schedule of master trust investments measured at fair value
Investments at fair value as of December 31, 2025 are summarized below:

December 31, 2025
Fair Value Measurements Using
(in thousands)Quoted PricesTotal
In ActiveSignificant
Markets forOtherSignificant
Identical ObservableUnobservable
AssetsInputsInputs
(Level 1)(Level 2)(Level 3)
Interest bearing cash$73 $— $— $73 
Total assets$73 $— $— $73 
Investments in the Master Trust
Registered investment companies$97,747 $— $— $97,747 
Equity securities2,307,397 — — 2,307,397 
Fixed income securities— 406,994 — 406,994 
Self-directed brokerage accounts459,800 — — 459,800 
Merck common stock907,977 — — 907,977 
Investments measured within the fair value hierarchy$3,772,921 $406,994$— $4,179,915 
Common/collective trusts measured at NAV practical expedient (1)
$12,677,091 
Master trust investments at fair value$16,857,006 

(1) Certain investments that were measured at net asset value (NAV) per share or its equivalent have not been classified in the fair value hierarchy. The fair value amounts presented in this table also do not include the Master Trust's accrued interest and dividends or other net assets.

The Plan's interests within the Master Trust as specifically identified in Note 6 are leveled in the same manner as the Master Trust investment categories presented above.
Investments at fair value as of December 31, 2024 are summarized below:
December 31, 2024
Fair Value Measurements Using
(in thousands)Quoted PricesTotal
In ActiveSignificant
Markets forOtherSignificant
Identical ObservableUnobservable
AssetsInputsInputs
(Level 1)(Level 2)(Level 3)
Interest bearing cash$43 $— $— $43 
Total assets$43 $— $— $43 
Investments in the Master Trust
Registered investment companies$500,106 $— $— $500,106 
Equity securities2,244,340 — — 2,244,340 
Fixed income securities— 386,182 — 386,182 
Self-directed brokerage accounts368,784 — — 368,784 
Merck common stock1,026,831 — — 1,026,831 
Investments measured within the fair value hierarchy$4,140,061 $386,182 $— $4,526,243 
Common/collective trusts measured at NAV practical expedient (1)
$11,107,463 
Master trust investments at fair value$15,633,706 

(1) Certain investments that were measured at net asset value (NAV) per share or its equivalent have not been classified in the fair value hierarchy. The fair value amounts presented in this table also do not include the Master Trust's accrued interest and dividends or other net assets.

The Plan's interests within the Master Trust as specifically identified in Note 6 are leveled in the same manner as the Master Trust investment categories presented above.
v3.26.1
Description of Plan (Details) - EBP 061
 12 Months Ended
Dec. 31, 2025
USD ($)
EBP, Description of Plan [Line Items]  
Participant contribution, maximum contribution to eligible compensation, percentage 25.00%
Participant contribution, pretax, maximum annual compensation to total compensation, percentage 100.00%
Employer contribution, matching percentage 4.50%
Participant contribution, aftertax, maximum annual compensation to total compensation, percentage 25.00%
Employer contribution, matching percentage per US dollar $ 0.75
Employer contribution, participant compensation matched, percentage 6.00%
Participant contribution, provision period before deferrals begin 60 days
Participant contribution, automatic, deferral rate 6.00%
Basis spread on variable rate 1.00%
Short term loan, borrowing term 1 year
Receivable from participant, account, primary residence, maximum borrowing term 15 years
Receivable from participant, account, minimum borrowing, amount $ 1,000
Receivable from participant, account, maximum borrowing, amount $ 50,000
Note receivable from participant, account, maximum borrowing, to total account, percentage 50.00%
Minimum  
EBP, Description of Plan [Line Items]  
Participant contribution, automatic, deferral rate, annual automatic increase 1.00%
Short term loan, borrowing term 1 year
Notes receivable from participants, interest rate 4.25%
Maximum  
EBP, Description of Plan [Line Items]  
Participant contribution, automatic, deferral rate, annual automatic increase 3.00%
Short term loan, borrowing term 5 years
Notes receivable from participants, interest rate 9.50%
v3.26.1
Related-Party Transactions (Details) - EBP 061 - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
EBP, Related Party and Party-in-Interest Transactions [Line Items]    
        Investment in Master Trust, at fair value $ 73 $ 43
Trustee    
EBP, Related Party and Party-in-Interest Transactions [Line Items]    
        Investment in Master Trust, at fair value 8,500 7,000
Increase from dividend income on investment 40  
Realized gain on investment 350  
Unrealized gain on investment 2,670  
Merck common stock    
EBP, Related Party and Party-in-Interest Transactions [Line Items]    
        Investment in Master Trust, at fair value 29,300 $ 29,500
Increase from dividend income on investment 900  
Realized gain on investment 1,400  
Unrealized gain on investment 100  
Purchase price 1,000  
Sale of common stock $ 2,700  
v3.26.1
Master Trust - Schedule of Master Trust Net Assets (Details) - EBP 061 - USD ($)
$ in Thousands
Dec. 31, 2025
Dec. 31, 2024
EBP, Master Trust [Line Items]    
Investments, at fair value $ 244,610 $ 228,820
Net assets available for benefits 246,925 231,602
Master Trust    
EBP, Master Trust [Line Items]    
Investments, at fair value 16,857,006 15,633,706
Accrued interest and dividends 15,877 17,180
Other net assets 8,648 4,994
Net assets available for benefits 16,881,531 15,655,880
Master Trust | Registered investment companies (mutual funds)    
EBP, Master Trust [Line Items]    
Investments, at fair value 97,747 500,106
Master Trust | Common/collective trusts    
EBP, Master Trust [Line Items]    
Investments, at fair value 12,677,091 11,107,463
Master Trust | Equity securities    
EBP, Master Trust [Line Items]    
Investments, at fair value 2,307,397 2,244,340
Master Trust | Fixed income securities    
EBP, Master Trust [Line Items]    
Investments, at fair value 406,994 386,182
Master Trust | Self-directed brokerage accounts    
EBP, Master Trust [Line Items]    
Investments, at fair value 459,800 368,784
Master Trust | Merck common stock    
EBP, Master Trust [Line Items]    
Investments, at fair value 907,977 1,026,831
Puerto Rico    
EBP, Master Trust [Line Items]    
Accrued interest and dividends 389 388
Other net assets 143 70
Net assets available for benefits 244,537 228,777
Puerto Rico | Registered investment companies (mutual funds)    
EBP, Master Trust [Line Items]    
Investments, at fair value 1,674 7,602
Puerto Rico | Common/collective trusts    
EBP, Master Trust [Line Items]    
Investments, at fair value 171,262 151,225
Puerto Rico | Equity securities    
EBP, Master Trust [Line Items]    
Investments, at fair value 33,498 32,895
Puerto Rico | Fixed income securities    
EBP, Master Trust [Line Items]    
Investments, at fair value 7,217 6,655
Puerto Rico | Self-directed brokerage accounts    
EBP, Master Trust [Line Items]    
Investments, at fair value 1,048 431
Puerto Rico | Merck common stock    
EBP, Master Trust [Line Items]    
Investments, at fair value $ 29,306 $ 29,511
v3.26.1
Master Trust - Schedule of Net Investment Gain (Details) - EBP 061
$ in Thousands
 12 Months Ended
Dec. 31, 2025
USD ($)
EBP, Master Trust [Line Items]  
    Net appreciation $ 33,756
Master Trust  
EBP, Master Trust [Line Items]  
    Net appreciation 2,354,281
    Interest and dividends 157,490
        Total investment income $ 2,511,771
v3.26.1
Fair Value Measurements (Details) - EBP 061 - USD ($)
$ in Thousands
Dec. 31, 2025
Dec. 31, 2024
EBP, Master Trust [Line Items]    
Master trust investments at fair value $ 244,610 $ 228,820
Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 16,857,006 15,633,706
Registered investment companies | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 97,747 500,106
Equity securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 2,307,397 2,244,340
Fixed income securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 406,994 386,182
Self-directed brokerage accounts | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 459,800 368,784
Merck common stock | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 907,977 1,026,831
Common/collective trusts | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 12,677,091 11,107,463
Fair Value, Inputs, Level 1, Level 2, and Level 3 | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 4,179,915 4,526,243
Fair Value, Inputs, Level 1, Level 2, and Level 3 | Interest bearing cash | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 73 43
Fair Value, Inputs, Level 1, Level 2, and Level 3 | Registered investment companies | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 97,747 500,106
Fair Value, Inputs, Level 1, Level 2, and Level 3 | Equity securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 2,307,397 2,244,340
Fair Value, Inputs, Level 1, Level 2, and Level 3 | Fixed income securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 406,994 386,182
Fair Value, Inputs, Level 1, Level 2, and Level 3 | Self-directed brokerage accounts | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 459,800 368,784
Fair Value, Inputs, Level 1, Level 2, and Level 3 | Merck common stock | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 907,977 1,026,831
Quoted Prices In Active Markets for Identical Assets (Level 1) | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 3,772,921 4,140,061
Quoted Prices In Active Markets for Identical Assets (Level 1) | Interest bearing cash | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 73 43
Quoted Prices In Active Markets for Identical Assets (Level 1) | Registered investment companies | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 97,747 500,106
Quoted Prices In Active Markets for Identical Assets (Level 1) | Equity securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 2,307,397 2,244,340
Quoted Prices In Active Markets for Identical Assets (Level 1) | Fixed income securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Quoted Prices In Active Markets for Identical Assets (Level 1) | Self-directed brokerage accounts | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 459,800 368,784
Quoted Prices In Active Markets for Identical Assets (Level 1) | Merck common stock | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 907,977 1,026,831
Significant Other Observable Inputs (Level 2) | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 406,994 386,182
Significant Other Observable Inputs (Level 2) | Interest bearing cash | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Other Observable Inputs (Level 2) | Registered investment companies | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Other Observable Inputs (Level 2) | Equity securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Other Observable Inputs (Level 2) | Fixed income securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 406,994 386,182
Significant Other Observable Inputs (Level 2) | Self-directed brokerage accounts | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Other Observable Inputs (Level 2) | Merck common stock | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Unobservable Inputs (Level 3) | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Unobservable Inputs (Level 3) | Interest bearing cash | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Unobservable Inputs (Level 3) | Registered investment companies | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Unobservable Inputs (Level 3) | Equity securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Unobservable Inputs (Level 3) | Fixed income securities | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Unobservable Inputs (Level 3) | Self-directed brokerage accounts | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
Significant Unobservable Inputs (Level 3) | Merck common stock | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value 0 0
NAV | Common/collective trusts | Master Trust    
EBP, Master Trust [Line Items]    
Master trust investments at fair value $ 12,677,091 $ 11,107,463
v3.26.1
Schedule H, Line 4i – Schedule of Assets (Held At End of Year) (Details) - EBP 061
 12 Months Ended
Dec. 31, 2025
USD ($)
EBP, Schedule of Asset Held for Investment [Line Items]  
Master trust, current value $ 244,537,069
EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value 73,226
Notes receivable from participants, current value 2,267,542
Total current value $ 246,877,837
Minimum  
EBP, Schedule of Asset Held for Investment [Line Items]  
Notes receivable from participants, interest rate 4.25%
Maximum  
EBP, Schedule of Asset Held for Investment [Line Items]  
Notes receivable from participants, interest rate 9.50%