How do antidepressants affect breastfeeding? (supply, side effects)

Depends: Most if not all antidepressants have not been well studied in lactation let alone pregnancy and are usually category c in pregnancy. In other words, not enough information to give a good answer so probably best not to use unless specifically told to by your doctor. Most warnings say possibly to probably unsafe with some of the other older agents stating possibly safe. Check with your doctor.

Secreted milk: Most antidepressants are secreted in milk and they don't seem to interfere with supply of milk. They do not cause any physical side effects to the baby. However it is unknown if they cause any long term effects on baby.