How long do you need to use a backup method while on antibiotics? I was on them for 10 days. When I saw my Dr she didn't say anything about a backup method with my bc and neither did the pharmacy but Its fairly common knowledge that antibiotics can make birth control ineffective. I'm just not sure about thr specifics. Problem is I didn't even think of this until after the fact. I'm not too worried but I'm wondering for the future