I was on an antibiotic a few weeks ago (Cefzil) and asked my pharmacist about this. One of the ones he said DOES lower the effectiveness of BC pills is tetracycline. Before I was on BC pills my boyfriend and I used the pull-out method and I got pregnant in April, so please don't keep relying on that. I'd use a condom from now on and hopefully you will get your period. I guess in a situation like that you have to pick one or the other - antibiotic or BC pills.