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. They are not absorbed from the stomach or intestines. These include many drugs which are given by injection.
Examples are gentamicin, heparin, lidocaine or other local anaesthetics used by dentists.
Medications applied to the skin, inhaled or applied to the eyes or nose are almost always safe for breastfeeding. You can still breastfeeding after general, regional or local anaesthesia. As soon as you are up to it. Medications you might take afterwards for pain are almost always permitted. Get reliable information before stopping breastfeeding.