At usual doses, minoxidil is considered compatible with breastfeeding.
Topical medication seems to be of low risk to the baby. However, systemic administration should be used with caution when administered to mothers of premature or is used for a long time or in high doses, since not enough information is available to support its therapeutic use and there is not sufficient published observational data supporting its safety.
Warning of the manufacturer:
Because orally administered minoxidil is excreted in breast milk is not recommended for use during breastfeeding.
This warning includes topical minoxidil and specialties for systemic application.