Though if you have cancer, you may need to start treatment as soon as possible, in which case you'll have to give up breastfeeding. Also avoid street drugs such as marijuana, cocaine, and heroin. Aside from these three categories of drugs, very few medications are likely to decrease your milk supply or harm your baby.
Many people ask about the safety of birth control pills.
The AAP considers them safe during breastfeeding. However, in a few women they may decrease the milk supply, especially if they are started earlier than three to four months postpartum. Many people assume antidepressants are incompatible with breastfeeding. In truth, some antidepressants are safe for nursing moms. If your doctor recommends one, tell her you're nursing so she can prescribe an appropriate brand.