Drug Safety in Lactation

Sharon Gardiner and Evan Begg

Prescriber Update No.21:10-23

Many mothers are required to use drugs during breastfeeding. Almost all drugs transfer into breast milk and this may carry a risk to a breastfed infant. Factors such as the dose received via breast milk, and the pharmacokinetics and effect of the drug in the infant need to be taken into consideration. Problems should not be overstated however, as many drugs are considered ‘safe’ during breastfeeding.

Transfer of drugs into breast milk is influenced by protein binding, lipid solubility and ionisation

Nearly all drugs transfer into breast milk to some extent. Notable exceptions are heparin and insulin which are too large to cross biological membranes. The infant almost invariably receives no benefit from this form of exposure and is considered to be an ‘innocent bystander’. Drug transfer from maternal plasma to milk is, with rare exceptions, by passive diffusion across biological membranes. Transfer is greatest in the presence of low maternal plasma protein binding and high lipid solubility. In addition, milk is slightly more acidic than plasma (pH of milk is approximately 7.2 and plasma is 7.4) allowing weakly basic drugs to transfer more readily into breast milk and become trapped secondary to ionisation. Milk composition varies within and between feeds and this may also affect transfer of drugs into breast milk. For example, milk at the end of a feed (hindmilk) contains considerably more fat than foremilk and may concentrate fat-soluble drugs.