During the time you are breast feeding, you must continue to be as careful about taking medications as you were during pregnancy. Anything you take will cross into your breast milk.
Follow this general principle: If you do not have to take medication, don't. If you have to take medication, make sure your health worker knows that you are breast feeding, and ask her or him to explain the possible effects on the baby. Many drugs are compatible with breast feeding. For example, if you are diabetic and take insulin every day, you can certainly continue to breast feed. But make sure your health worker knows you are breast feeding and helps you adjust your insulin dose accordingly.