WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• The extent of drug transfer into milk during recreational intravenous use of methylamphetamine has not previously been studied.WHAT THIS STUDY ADDS• We have shown that methylamphetamine transfers into breast milk.• The amount a breastfed infant would receive varied over 2.5-fold range.• A 48-h withholding period for breastfeeding is recommended following recreational use.AIMS To investigate the transfer of amphetamines into breast milk following their recreational use and estimate drug exposure for the breastfed infant.METHODS Two breastfeeding mothers who were occasional recreational users of intravenous amphetamines were studied. A urine sample was collected 4 h after dose, and milk samples were collected over 24 h. Drug in urine was qualitatively identified by gas chromatography-mass spectrometry and quantification in milk was by high-performance liquid chromatography. Absolute infant dose via milk was estimated.RESULTS The urines contained predominantly methylamphetamine together with smaller amounts of amphetamine. In the 24 h after dose, average concentrations in milk were 111 µg l−1 and 281 µg l−1 for methylamphetamine and 4 µg l−1 and 15 µg l−1 for amphetamine in cases 1 and 2, respectively. Absolute infant doses for methylamphetamine plus amphetamine (as methylamphetamine equivalents) were 17.5 µg kg−1 day−1 and 44.7 µg kg−1 day−1, respectively, for cases 1 and 2.CONCLUSION These limited data suggest that breastfeeding should be withheld for 48 h after recreational amphetamine use.