Depression and breastfeeding

I have always struggled with depression and have been off of medication since just before becoming pregnant. I have maintained ok during most of my pregnancy with of course some harder days in the mix. I told my sister tonight I wasn't sure I would be able to breast feed because I will probably have to go back on medication right after baby comes to hopefully stave off post partum. She seemed completely disgusted with me and offended by my possible choice. What would or has anyone done in a similar situation? My sister's judgement aside, I wonder if it is possible or safe to breast feed while on medication, or has anyone foregone the medication to breastfeed? Any information I have found has seemed contradicting. Any advice is appreciated.