Transfer of Dicloxacillin into Human Milk

Dicloxacillin is a beta-lactam antibiotic that is commonly used in the treatment of lactational mastitis in breastfeeding women. Although penicillins have long been considered safe for breastfeeding mothers and their infants, there is almost no data on the transfer of dicloxacillin into human breast milk despite the fact that it is commonly used for mastitis. This study determined the drug concentration-time profile of dicloxacillin in milk samples collected from three lactating mothers consuming 500 mg dicloxacillin taken every 6 hours for treatment of mastitis. Milk levels were measured using liquid chromatography mass spectrometry. The maximum concentration of dicloxacillin in milk was 67.6 ng/mL. The relative infant dose (RID) was calculated to be 0.03%. This value is well below the theoretical level of concern of 10%. The limited transfer of dicloxacillin into human milk is probably explained by the high plasma protein binding of dicloxacillin and its subsequent poor penetration into human milk. In this case series, the level of dicloxacillin in milk was found to be very low, and the RID to be only 0.03% of the maternal dose. Although the levels detected were low, dicloxacillin does transfer into breast milk. Caution should be exercised in infants with hypersensitivity to penicillins.