I know it's possible to get pregnant when you're on birth control but also taking antibiotics... Well, i've been on penicilin for roughly a week and still have a few more doses to take. I've been having sex without a condom, my boyfriend doesn't come in me (and hasn't in a long time) and he also never seems to have pre-come (who knows, though). Anyone know how long you need to use a backup method to be protected because of the antibiotics or if penicillin really is one of the antibiotics that decreases the effect of birth control? I'm on ortho lo. I've read conflicting things. Kimmeh, i'm pretty sure you said you got preg this way... Let me know... Thanks girls