If you are on bc there is always a chance that it is less effective when taking antibiotics. Your pharmacist should have informed you of this either by telling you or there should have been a sticker on your bc packet and/or the anti-biotics. (Z-paks or Tri-paks stay in your system for a total of 7 days) As long as your being careful you are fine. Sounds like you took precaution by taking plan b. I honestly don't know a whole lot about plan b because we don't usually dispense it at the pharmacy I work in. But I beleive you are to take it within 48 hours of unprotected sex. I don't know if you really needed to take it but I guess it probably makes you feel better. It may screw up your next period so just be prepared. Hope this helps a little! I don't claim to know it all but I'll tell you what I do know!

Yes, birth control becomes less effective with antibiotics. However, you did find out within 72 hours and took Plan B which is good. It is really hard to tell your chances of pregnancy, though. Since Plan B works to prevent ovulation, the sooner to the sex act it is taken the more effective it is. If you happened to ovulate during those two days before you took Plan B, unfortunately the Plan B would not do too much to help. However, if you haven't ovulated yet then it should help you out a lot. You are going to have to just wait it out and test in 14 days. Good luck!