Most drugs taken by the mother reach breast milk and are ingested by the nursing infant. Most of the drugs are detected in breast milk at low concentrations, so breastfeeding by women taking these drugs is possible. The effect of some drugs on the nursing infant is unknown and further studies are needed. Some drugs may achieve significant infant plasma concentrations and may be unsafe for the infant. In these cases the infant should be carefully monitored for any clinical side effects and whenever observed, breastfeeding should be discontinued. Despite the confirmed benefits of breastfeeding, there are certain drugs which are absolutely contraindicated; in these cases interruption of breastfeeding is necessary. This review summarises the current scientific knowledge on compatibility of drugs with breastfeeding, focusing on drugs that are contraindicated and of which use in breastfeeding remains controversial.