FDA updates Mounjaro and Zepbound breastfeeding labels after required milk study
The September 25 approvals rewrite the breastfeeding information for Mounjaro and Zepbound using data from a milk study, though the measured levels were not disclosed in the approval letters.
The FDA approved changes to the breastfeeding information for Mounjaro and Zepbound on September 25, 2025, after the company that makes both drugs submitted results from a study measuring tirzepatide in breast milk [1][2].
The two approvals cover supplements to the drugs' applications: NDA 215866/S-034 for Mounjaro and NDA 217806/S-031 for Zepbound [1][2]. Both supplements were dated and received March 31, 2025 [1][2]. Each letter says the supplement revises subsection 8.2, Lactation, of the prescribing information and revises the medication guide for Mounjaro and Zepbound [1][2]. Both letters were signed by John Sharretts, MD, director of the Division of Diabetes, Lipid Disorders, and Obesity in the FDA's Center for Drug Evaluation and Research, on September 25, 2025 [1][2].
The labeling changes rest on a study called I8F-MC-GPIN, described in the letters as "A Study to Evaluate Tirzepatide Concentrations in Breastmilk Following Administration of Single Dose of Tirzepatide by Subcutaneous Injection in Healthy Lactating Females" [1][2]. That study was run to satisfy a postmarketing requirement, PMR 4272-2, listed in the May 13, 2022, approval letter for Mounjaro: "Conduct a milk-only lactation study in lactating women who have received a dose of tirzepatide to assess concentrations of tirzepatide in breast milk using a validated assay" [1][2]. The FDA told Eli Lilly that the requirement has been fulfilled [1].
The approval letters do not report the concentrations the study measured, and they do not include the new lactation wording. Those details have not been disclosed in the letters, so what the revised label says about how much tirzepatide passes into milk is not yet known from these documents. The study title also describes a single dose given to healthy lactating females; the letters do not describe data from repeated dosing [1][2].
The FDA also reminded Lilly that other postmarketing requirements tied to the May 13, 2022, approval letter and a November 29, 2024, new postmarketing requirement letter remain open [1].
Why it matters for patients
People who are breastfeeding while using tirzepatide now have a labeling section built on a study designed to measure the drug in breast milk [1][2]. The revised language applies to the prescribing information for both brand-name products and to their medication guides, which are the plain-language handouts that come with the drugs [1][2].
The letters do not say whether the revised labels support or discourage breastfeeding during treatment, and they do not give the measured milk levels [1][2]. Patients and clinicians will need to read the published labeling to see what changed.
Because the study looked at a single dose, the letters do not address whether milk levels differ with repeated dosing over time [1][2].
What happens next
Within 14 days of the September 25, 2025, letters — by about October 9, 2025 — Lilly must submit the updated labeling in structured product labeling format, after which it becomes accessible from publicly available labeling repositories [1][2].
Postmarketing requirements listed in the May 13, 2022, approval letter and the November 29, 2024, letter remain open, and the FDA said it still expects Lilly to meet them [1].
Sources
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