I am on the antibiotic Amoxicillan for a gum infection, I take Levlan birth control. I know that the antibiotic can make the birth control not work right. How long do I need to use backup so I don't get pregnant?? I just started taking the antibiotic yesterday (Monday). I have started to have some breakthrough bleeding today and I am guessing it is from the antibiotic, can anyone help me??

You definitely need to use backup the entire time you are taking the antibiotic. Once you finish the antibiotics, you may need to use back up for 7 days to a month, depending on the specific antibiotic and its interactions with the pill. Ask your doctor. I imagine that you should only need the backup for a week or two after you stop the antibiotics.