默沙东美股招股说明书(2026-05-20版)

We are offering $500,000,000 aggregate principal amount of ourFloating Rate Notes due 2028 (the “floating rate notes”), $1,000,000,000 aggregate principal amount of our 4.300%Notes due 2028 (the “2028 notes”), $500,000,000 aggregate principal amount of our 4.650% Notes due 2031 (the “2031 notes”), $1,000,000,000 aggregate principal amount of our 4.950%Notes due 2033 (the “2033 notes”), $1,500,000,000 aggregate principal amount of our 5.200% Notes due 2036 (the “2036 notes”), $500,000,000 aggregate principal amount of our 5.750%Notes due 2046 (the “2046 notes”) and $1,000,000,000 aggregate principal amount of our 5.850% Notes due 2056 (the “2056 notes”). We refer to the 2028 notes, the 2031 notes, the 2033notes, the 2036 notes, the 2046 notes and the 2056 notes collectively as the “fixed rate notes.” We refer to the fixed rate notes and the floating rate notes collectively as the “notes.” Interest on the floating rate notes is payable on February 22, May 22, August 22 and November 22 of each year, beginning onAugust 22, 2026, and interest on the fixed rate notes ispayable on May 22 and November 22 of each year, beginning onNovember 22, 2026. The floating rate notes will bear interest at a floating rate equal to Compounded SOFR (as defined herein)plus0.370%, subject to the provisions set forth under “Description of the Notes—Interest—Floating Rate Notes.” The fixed rate notes of each series will bear interest at the annual interest rateshown above for such series of notes. The floating rate notes will mature onMay 22, 2028, the 2028 notes will mature onMay 22, 2028, the 2031 notes will mature onMay 22, 2031, the 2033notes will mature onMay 22, 2033, the 2036 notes will mature onMay 22, 2036, the 2046 notes will mature onMay 22, 2046 and the 2056 notes will mature onMay 22, 2056. We may redeem some or all of the fixed rate notes of each series at any time at the applicable redemption price set forth in this prospectus supplement under the caption “Description ofthe Notes—Optional Redemption.” The floating rate notes are not redeemable prior to maturity. On March 25, 2026, we announced a definitive agreement (the “Merger Agreement”) to acquire Terns Pharmaceuticals, Inc. (“Terns”) through a subsidiary (the “Terns Acquisition”). TheTerns Acquisition closed on May 5, 2026. In connection with the closing of the Terns Acquisition, we borrowed funds under a 364-Day Delayed Draw Term Loan Credit Agreement, dated as ofApril 1, 2026 (the “Credit Agreement”), to finance a portion of the consideration for the Terns Acquisition and related fees and expenses. We intend to use the net proceeds of this offering torepay in full all outstanding borrowings under the Credit Agreement. See “Use of Proceeds.” The notes will be our unsecured senior debt obligations and will rank equally with all of our other unsecured senior indebtedness from time to time outstanding. The notes will be issuedonly in denominations of $2,000 and integral multiples of $1,000 in excess thereof. The notes will not be convertible or exchangeable. Investing in the notes involves risks. See “Risk Factors” beginning on pageS-3of this prospectus supplement and in the documents incorporated by reference in thisprospectus supplement and the accompanying prospectus. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved ofthese securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense. Per floating rate noteTotalPer 2028 noteTotalPer 2031 noteTotalPer 2033 noteTotalPer 2036 noteTotalPer 2046 noteTotalPer 2056 noteTotal Citigroup J.P. Morgan BNP PARIBASBBVABlaylock Van, LLC TABLE OF CONTENTS Prospectus Supplement About This Prospectus SupplementMerck & Co., Inc., Rahway, N.J., USARisk FactorsForward-Looking StatementsUse of ProceedsCapitalizationDescription of the NotesCertain U.S. Federal Tax ConsequencesUnderwriting(Conflicts of Interest)Incorporation of Certain Documents by ReferenceValidity of the NotesExperts Prospectus About This ProspectusMerck & Co., Inc., Rahway, N.J., USAMSD Netherlands Capital B.V.Risk FactorsForward-Looking StatementsUse of ProceedsDescription of Debt Securities Parent May OfferDescription of Debt Securities MSD Netherlands May OfferLegal Ownership and Book-Entry IssuancePlan of DistributionValidity of Debt SecuritiesExpertsWhere You Can Find More InformationIncorporation of Certain Documents by ReferenceEnforcement of Judgments ABOUT THIS PROSPECTUS SUPPLEMENT We have not, and the underwriters have not, authorized anyone to provide you with any information other than thatcontained or incorporated by reference in this prospectus supplement, any related free writing prospectus prepared by us orthe accompanying prospectus. We take no responsibility for, and can provide no assurance as to the reliability of any otherinformation that others may give you. If t