I have been taking ortho tri-cyclen lo for over 3 years. On the 7th I was diagnosed with a bacterial infection and given 7 days worth of flagyl (metronidazole). I had unprotected sex multiple times during the week I was taking flagyl, and my boyfriend came inside me multiple times. I was also on my period during this time. So my question is- should I be worried about possibly being pregnant?

22. August 2012


The effectiveness of any type of estrogen-containing birth control pill may be impaired during treatment with some types of antibiotics.  However, the risk is very slight, and the supportive evidence is primarily limited to anecdotes from case reports and findings from uncontrolled or poorly controlled studies.  In fact, according to Contraceptive Technology, 19th Edition, it is not necessary to use a back up method of birth control while on antibiotics because they do not lower the levels of hormones in your blood to the point that you would be at risk for pregnancy. The only exception is the medication, rifampin (which is primarily used to treat tuberculosis).   To be on the safe side, however, we do recommend that our pill-users also use condoms or abstain from sex during antibiotic treatment and for at least one week after.

As long as you have been taking your birth control pill at the same time every day, pregnancy is highly unlikely, especially since the sex occurred during your period. If you are a week or more late for your next period, take a pregnancy test. If it is negative, repeat the test two weeks later if you still haven’t had any bleeding. Do not stop taking the pill unless you test positive for pregnancy. You should also be aware that some medications can cause menstrual irregularities so don’t panic if your next period comes later than normal or is lighter or heavier than usual.

For future reference, the following drugs or herbal products can decrease the effectiveness of the pill:

  • barbiturates
  • bosentan
  • carbamazepine
  • felbamate
  • griseofulvin
  • oxcarbazepine
  • phenytoin
  • rifampin
  • St. John’swort
  • topiramate
  • HIV protease inhibitors or non-nucleoside reverse transcriptase inhibitors

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