Most drugs appear in the milk, but only in very tiny amounts. Although a very few drugs may still cause problems for infants even in tiny doses, this is not the case for the vast majority. Mothers who are told they must stop breastfeeding because of a certain drug should ask to be prescribed an alternative medication which is acceptable for breastfeeding mothers. In this day and age, it is rarely a problem to find such an alternative. If the prescribing physician is not flexible, the mother should seek another opinion.
Examples are amoxycillin, cloxacillin, most antibiotics.
Drugs enter directly into the baby's bloodstream when used during pregnancy. The baby generally gets much higher doses at a much more sensitive period during pregnancy, than during breastfeeding. This is not an absolute, however, as during pregnancy, the mother's liver and kidneys will get rid of the drug for the baby. These include many drugs which are given by injection. Examples are gentamicin, heparin, lidocaine or other local anaesthetics used by dentists.