Should you breastfeed while taking medicines, can those medicines pass through breastmilk and harm the baby?

It's always best to tell your doctor or pharmacist that you're breastfeeding, when discussing medicines. 

Small amounts of any medicine you take may or may not pass through your breast milk to your baby. Generally, the amounts are very low and very few medicines are unsafe while you're breastfeeding but its crucial to talk to your doctor. (Source: NHS)