FDA updates tirzepatide breastfeeding labeling

The FDA on September 25, 2025, approved matching label updates to the Lactation subsection of Mounjaro and Zepbound, both brand names for the drug tirzepatide made by Eli Lilly ^[1][2]. The changes came from a supplemental new drug application Lilly filed on March 31, 2025, and were approved for both products on the same day ^[1][2].

The update is based on a study called I8F-MC-GPIN, which measured tirzepatide levels in breast milk after a single subcutaneous injection in healthy lactating women ^[1][2]. The FDA had required this study back in its original May 13, 2022, approval letter for Mounjaro, listed as Postmarketing Requirement 4272-2: "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]. With this submission, the FDA said it has reviewed the final report and "conclude[s] that the above requirement has been fulfilled" ^[1].

The approval revised subsection 8.2 (Lactation) of the Prescribing Information and also updated the Medication Guide that accompanies both Mounjaro and Zepbound ^[1][2]. Because the underlying study is the same for both drugs, the labeling changes are identical for the two brands ^[2]. The FDA classified both applications as "Prior Approval" supplements, meaning the changes had to be reviewed and cleared before Lilly could put them into effect ^[1][2].

The sources provided do not include the actual new label text, so the specific breast milk concentration numbers from the study, or any new guidance language about breastfeeding while using tirzepatide, are not known from what's available here. The approval letters describe the regulatory process and confirm the study was completed, but they do not quote the resulting label language ^[1][2].

Why it matters for patients

Mounjaro is approved for type 2 diabetes and Zepbound for weight management, and both use the same active ingredient, tirzepatide. People who are breastfeeding and considering or already using either drug will find updated information in the official label about what is known regarding how the drug moves into breast milk. Before this study, the label's lactation information would have been based on more limited data, since human lactation studies for newer drugs are often not available at initial approval.

This kind of label update does not change who can be prescribed the drug or for what condition. It changes the information available to patients and doctors making decisions together about breastfeeding while on the medication. Because the source documents here are the FDA approval letters, not the label itself, readers who want the specific data on breast milk concentrations will need to consult the updated Prescribing Information and Medication Guide directly once those documents are publicly posted.

It's also worth noting the approval letters mention that other postmarketing requirements from Mounjaro's 2022 approval letter and a November 29, 2024, letter remain open ^[1]. This means the FDA is still awaiting additional studies on tirzepatide beyond the lactation question that was just resolved.

What happens next

Lilly was required to submit the finalized label content in structured product labeling format to the FDA within 14 days of the September 25, 2025, approval letters, meaning by around October 9, 2025 ^[1][2]. Once submitted, that labeling becomes accessible through public FDA labeling repositories ^[1][2]. Lilly must also submit related patent information to the FDA within 30 days of approval, by around October 25, 2025 ^[1][2]. The sources do not specify a date when the updated label will be visible to the public or posted to drug information sites.