So I've been on minastrin since July, taking every pill regularly, and I started bactrim 6 days ago (continue it for 4 more days) for a bacterial infection in my throat and I had sex. Should I resort to a back up method and buy some plan b? My doctor had told me that antibiotics decrease birth control a then said I wouldn't have to worry because bactrim wasn't going to do that but I still don't know. But looking online I don't know if they were just saying that or if I misunderstood.