默沙东美股招股说明书(2025-12-03版)

Merck & Co., Inc., Rahway, N.J., USA $500,000,000 Floating Rate Notes due 2029$750,000,0003.850% Notes due 2029$1,000,000,000 4.150% Notes due 2031$1,000,000,000 4.450% Notes due 2032$1,500,000,000 4.750% Notes due 2035$750,000,0005.500% Notes due 2046$1,500,000,0005.550% Notes due 2055 We are offering $500,000,000aggregate principal amount of our Floating Rate Notes due 2029 (the “floating rate notes”),$750,000,000aggregate principal amount of our 3.850% Notes due 2029 (the “2029 notes”), $1,000,000,000aggregate principalamount of our 4.150% Notes due 2031 (the “2031 notes”), $1,000,000,000 aggregate principal amount of our 4.450% Notes due 2032(the “2032 notes”), $1,500,000,000aggregate principal amount of our 4.750% Notes due 2035 (the “2035 notes”), $750,000,000aggregate principal amount of our 5.500% Notes due 2046 (the “2046 notes”), $1,500,000,000 aggregate principal amount of our Interest on the floating rate notes is payable on March 15, June 15, September 15 and December 15 of each year, beginningonMarch 15, 2026, interest on the 2029 notes, the 2031 notes and the 2046 notes is payable on March 15 and September 15 of eachyear, beginning onMarch 15, 2026, and interest on the 2032 notes, the 2035 notes, the 2055 notes and the 2065 notes is payable onJune 4 and December 4 of each year, beginning onJune 4, 2026 . The floating rate notes will bear interest at a floating rate equal toCompounded SOFR (as defined herein) plus0.570%, 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 rate shown above for such series ofnotes. The floating rate notes will mature onMarch 15, 2029, the 2029 notes will mature onMarch 15, 2029, the 2031 notes will 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 thisprospectus supplement under the caption “Description of the Notes—Optional Redemption.” The floating rate notes are not On November 14, 2025, we announced a definitive agreement (the “Merger Agreement”) to acquire Cidara Therapeutics, Inc.(“Cidara”) through a subsidiary (the “Cidara Acquisition”). We intend to use the net proceeds of this offering for general corporatepurposes including, without limitation, to repay outstanding indebtedness, as well as potentially to fund a portion of the cash This offering is not contingent on the consummation of the Cidara Acquisition, which, if completed, will occur subsequent to theclosing of this offering. However, if (i) the Cidara Acquisition is not consummated on or before the later of (x) May 13, 2026 or (y)such later date to which the end date under the Merger Agreement as in effect on the closing date of this offering may be extended inaccordance with the terms thereof (such later date, the “Special Mandatory Redemption End Date”), (ii) prior to the Special MandatoryRedemption End Date, the Merger Agreement is terminated or (iii) we otherwise notify the trustee (as defined herein) that we will notpursue the consummation of the Cidara Acquisition, we will be required to redeem the floating rate notes, the 2029 notes, the 2031 The notes will be our unsecured senior debt obligations and will rank equally with all of our other unsecured senior indebtednessfrom time to time outstanding. The notes will be issued only in denominations of $2,000 and integral multiples of $1,000 in excess Investing in the notes involves risks. See “Risk Factors” beginning on pageS-3of this prospectus supplement and in thedocuments incorporated by reference in this prospectus supplement and the accompanying prospectus. Neither the Securitiesand Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined We expect that delivery of the notes will be made to investors in book-entry form only through the facilities of The DepositoryTrust Company and its participants, including Clearstream Banking S.A. and Euroclear Bank SA/NV, on or aboutDecember 4, 2025. Prospectus Supplement About This Prospectus SupplementMerck & Co., Inc., Rahway, N.J., USARisk FactorsForward-Looking Statements About This ProspectusMerck & Co., Inc., Rahway, N.J., USAMSD Netherlands Capital B.V.Risk FactorsForward-Looking Statements 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 the information varies between this prospectus supplement and the accompanyingprospectus, the information in this prospectus supplement supersedes the information in the accompanying prospectus. We arenot makin