Almost everything which you eat or drink passes into the breast milk and enters the body of your baby.
If your doctor needs to prescribe you medicine, it is essential that you say you are breastfeeding! For many complaints there are also alternative methods of treatment which are compatible with breastfeeding.
Sometimes, however, it is necessary to take medication for a certain period of time which is harmful to the baby.
If this is case, you should pump your milk during this period and dispose of it. Pumping will maintain your milk production and you can continue to breastfeed without any problems as soon as you have finished your course of medication.
With respect to smoking, the same applies during the breastfeeding period as it did during pregnancy: You should obviously not do it! This is because the harmful substances inhaled when smoking a cigarette pass into the breast milk. Naturally the same applies to drugs of every kind.
If it is not absolutely necessary, you should not have your teeth treated with amalgam fillings. When amalgam corrodes, mercury is released!
If such a treatment is unavoidable, however, then follow the same principle as that for taking medication: Pump your breast milk and dispose of it. Speak with your dentist about when you can continue breastfeeding.