I got off my period the 27th and I'd been on an antibacterial from the 28th-3rd. My boyfriend and I used condoms almost every time during that time, twice was the pull out method (which has never failed us before). I was also on an antibacterial last month on the 1st and we didn't have sex until the 11th. I'm on the pill and sometimes don't take it on time, but it's never been a serious issue before. Both times I was on the antibacterial I got a yeast infection. And I still have the second one.