The MedMilk (Medication concentrations in human Milk) study investigates how selected medicines taken by breastfeeding women are transferred into human milk and whether this may affect the breastfed child. The study includes breastfeeding women who are already using prescription or over-the-counter medicines as part of their usual care. Participants provide samples of breast milk and urine and complete a questionnaire about maternal and infant health. The collected data will be used to quantify medicine concentrations in milk and estimate the relative infant dose. The study aims to contribute new data to support safer prescribing and more informed counselling during breastfeeding

The MedMilk (Medication concentrations in human Milk) study is a prospective, exploratory clinical study coordinated by the Department of Clinical Pharmacology, Bispebjerg and Frederiksberg Hospital, Denmark. The study is observational in nature and involves no experimental drug administration. Participants are breastfeeding women using medicines as part of their routine treatment.

Each participant will provide four small breast milk samples (1-20 mL each) collected at different time points in relation to maternal drug intake, one urine sample, and questionnaire data on maternal and infant health, breastfeeding patterns, and any infant symptoms potentially related to drug exposure. Samples are stored at -80 °C in a dedicated research biobank until analysis.

Drug concentrations in milk and urine will primarily be quantified using solid Phase Extraction coupled with UPLC-MS/MS. Data will be analyzed descriptively, and relative infant doses (RID) will be calculated. For drugs with sufficient sampling density.

Results will add to the limited body of systematic data on drug transfer into human milk and help inform clinical decision-making regarding medication use during breastfeeding.

Inclusion Criteria:

• Lactating women aged ≥18 years
• Current use of one or more prescription or over-the-counter (classified within the ATC system) drugs
• Able and willing to provide written informed consent

Exclusion Criteria:

• Current pregnancy during sample collection
• Known and clinically significant pharmacokinetic drug-drug interactions at the time of sampling

Participants will be lactating women recruited through obstetric, neonatal, and psychiatric departments, public health visiting nurses, and counselling services across Denmark. Recruitment will be coordinated by the Department of Clinical Pharmacology, Bispebjerg and Frederiksberg Hospital, Copenhagen. The study primarily targets women using prescription and over-the-counter medicines during breastfeeding and aims to represent real-world exposure patterns across all Danish regions.

Study protocol, SAP and ICF after protocol publication. Analytical code and summary results after completion of data collection and analyses.

Requests for data access can be directed to the principal investigator. Data sharing will require relevant approvals under Danish GDPR.