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Income Taxes
 9 Months Ended
Sep. 30, 2025
Income Tax Disclosure [Abstract]  
Income Taxes Income Taxes
The effective income tax rates of 14.2% and 13.3% for the third quarter and first nine months of 2025, respectively, reflect the favorable impacts of geographical mix of income and expense, as well as certain discrete items.
The effective income tax rate of 22.7% for the third quarter of 2024 reflects a 7.2 percentage point combined unfavorable impact of charges related to the acquisitions of EyeBio and MK-1045, which had minimal tax benefits. The effective income tax rate of 15.1% for the first nine months of 2024 reflects a 2.1 percentage point combined unfavorable impact of charges related to the acquisitions of Harpoon, EyeBio and MK-1045, which had minimal tax benefits. The effective income tax rate for the first nine months of 2024 also reflects a 1.6 percentage point favorable impact due to a $259 million reduction in reserves for unrecognized income tax benefits resulting from the expiration in June 2024 of the statute of limitations for assessments related to the 2019 federal tax return year.
While many jurisdictions in which Merck operates have adopted the global minimum tax provision of the Organization for Economic Cooperation and Development (OECD) Pillar 2, effective for tax years beginning in January 2024, it resulted in a minimal impact to the Company’s 2024 effective income tax rate due to the accounting for the tax effects of intercompany transactions. In addition, in July 2025, H.R.1 - One Big Beautiful Bill Act (OBBBA) was enacted into law, which had an immaterial impact to the effective tax rates for the third quarter and first nine months of 2025.
The Internal Revenue Service (IRS) is currently conducting examinations of the Company’s tax returns for the years 2017 and 2018, including the one-time transition tax enacted under the Tax Cuts and Jobs Act of 2017 (TCJA). In April 2025, Merck received Notices of Proposed Adjustment (NOPAs) that would increase the amount of the one-time transition tax on certain undistributed earnings of foreign subsidiaries by approximately $1.3 billion. In addition, the NOPAs included penalties of approximately $260 million. These amounts are exclusive of any interest that may be due. The Company disagrees with the proposed adjustments and will vigorously contest the NOPAs through all available administrative and, if necessary, judicial proceedings. It is expected to take a number of years to reach resolution of this matter. If the Company is ultimately unsuccessful in defending its position, the impact could be material to its financial statements. The statute of limitations for assessments with respect to the 2019 and 2020 federal tax return years expired in June 2024 (as noted above) and October 2024, respectively. The IRS is also currently conducting examinations of the Company’s tax returns for the years 2021 and 2022. In addition, various state and foreign examinations are in progress.