I posted a question earlier and explained my experience with plan b recently. I started reading up on how certain antibiotics can interefere with plan b. how long does it take for smz/tmp to leave yoyr system? I stopped using it on the 15th and then used plan b on the 18th. please help. will plan b be affected even though I stopped using the antibiotic a few daysbbefore? I still meed to take a pregnancy test.