I'm on a very very low dosage of septra. It's a preventative for kidney infections, and I have to stay on it for the next 6 months. When I went on the pill, the doctor told me to take my preventative at the opposite time of the day that I take the pill. However, my mom has told me today that I have to take the preventative before I go to bed now, which screws me up because I take the pill at 10 at night. Also when I went on the pill, I was given a list of medications that will make the pill less effective, and septra was not on the list. Do you think I should be ok if me and my boyfriend have sex? I'm thinking I will be alright, but i'm not 100% sure. Like I said, it's a very low dosage, so I don't see how it can interfere with anything - or at least i'm hoping it won't.