Data on medication concentrations in human milk and their potential effects on the breastfed infant remain limited, leaving clinicians with insufficient evidence to guide women who require medication while breastfeeding. The MedMilk study aims to systematically measure medication concentrations in human milk, calculate the relative infant dose (RID) and evaluate infant outcomes.

This observational clinical study will recruit breastfeeding women using prescription and over-the-counter medications. Women will provide milk and urine samples along with self-reported maternal and infant health data. The primary outcome is quantification of medication concentrations in human milk. Secondary outcomes include estimation of the RID based on measured milk concentrations, maternally reported infant symptoms and adverse events potentially related to medication exposure, and pharmacokinetically simulated infant plasma exposure. Analytical methods include solid-phase extraction liquid chromatography-tandem mass spectrometry for quantification combined with descriptive statistics and pharmacokinetic modelling. The study aims to recruit approximately 250 women across medication groups. For the most frequently used medications, we aim to include 20 participants per medication, with each participant contributing four milk samples and a urine sample. Biological samples will be stored in a biobank, and data in a secure database.

The study has been approved by the Regional Research Ethics Committee of the Capital Region of Denmark (Journal no. H-23062687) and the Danish Data Protection Agency (Ref. P-2023-14627). All participants will provide written informed consent before enrolment. Findings will be disseminated through peer-reviewed journal publications and presentations at relevant national and international scientific conferences.

NCT07346716.