Most drugs are safe if:

They are commonly prescribed for infants. The amount the baby would get through the milk is much less than he would get if given directly.

They are considered safe in pregnancy. This is not always true, since during the pregnancy, the mother’s body is helping the baby’s get rid of drug. Thus it is theoretically possible that worrisome accumulation of the drug might occur during breastfeeding when it wouldn’t during pregnancy (though this is probably rare). However, if the concern is for the baby’s getting exposed to a drug, say an antidepressant, then the baby is getting exposed to much more drug at a much more sensitive time during pregnancy than during breastfeeding. Recent studies about withdrawal symptoms in newborn babies exposed to SSRI type antidepressants (Paxil, for example) during the pregnancy somehow managed to implicate breastfeeding as if this type of problem requires a mother not to breastfeed. (Good example of how breastfeeding is blamed for everything.) In fact, you cannot prevent these withdrawal symptoms in the baby by breastfeeding, because the baby gets so little in the milk.

They are not absorbed from the stomach or intestines. These include many, but not all, drugs given by injection. Examples are gentamicin (and other drugs in this family of antibiotics), heparin, interferon, local anaesthetics, omeprazole. Omeprazole (Losec, Prilosec) is interesting because it is destroyed very quickly in the stomach. During the manufacture of the drug, a protective layer is added to the drug to prevent its destruction and the drug is thus absorbed into the mother’s body. Thus, the drug is covered by a protective layer that prevents its destruction in the stomach. However, when the baby gets the drug (in tiny amounts incidentally) there is no protective layer on the drug, so it is immediately destroyed in the baby’s stomach.

They are not excreted into the milk. Some drugs are just too big to get into the milk. Examples are heparin, interferon, insulin, infliximab (Remicade), etanercept (Enbrel).