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Income Taxes
 6 Months Ended
Jun. 30, 2025
Income Tax Disclosure [Abstract]  
Income Taxes Income Taxes
The effective income tax rates of 11.4% and 12.7% for the second quarter and first six months of 2025, respectively, reflect a 2.9 percentage point favorable impact and a 1.4 percentage point favorable impact, respectively, due to $146 million of tax benefits resulting primarily from favorable audit adjustments. The effective income tax rates in both the second quarter and first six months of 2025 also reflect the favorable impacts of geographical mix of income and expense, as well as certain discrete items.
The effective income tax rates of 9.1% and 12.4% for the second quarter and first six months of 2024, respectively, reflect a 4.3 percentage point favorable impact and a 2.2 percentage point favorable impact, respectively, 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. The effective income tax rate for the first six months of 2024 also reflects a 0.7 percentage point unfavorable impact of a charge for the acquisition of Harpoon for which no tax benefit was recognized.
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. The Company expects the impact of the global minimum tax to be approximately 2% for full year 2025. In addition, in July 2025, H.R.1 - One Big Beautiful Bill Act (OBBBA) was enacted into law. The Company is currently evaluating the effects of the OBBBA but does not expect a material tax impact.
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 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.