Antibiotics and Birth Control Pills

Birth Control Pill Failure

Most birth control pills are classified as combined oral contraceptives. These pills contain two different hormones -- an estrogen and a progestin. There are also oral contraceptives that only contain a progestin and no estrogen. These are known as progestin-only contraceptives.
Progestin-only contraceptives are not as effective at preventing pregnancies as the combined oral contraceptives. When most people use the term "the pill," they are referring to combined oral contraceptives.
Combined oral contraceptives can decrease the risk of becoming pregnant to less than 1 percent when used consistently and without missing any pills. However, not everyone can take birth control pills perfectly all the time, so the actual chance of becoming pregnant with typical (not perfect) use ranges between 1 and 5 percent. For progestin-only contraceptives, the chance of becoming pregnant with typical use is around 8 percent.
Clinical studies have focused on determining whether taking an antibiotic while on an oral contraceptive puts women at a higher risk of experiencing a birth control failure. While a certain percentage of women on the pill did get pregnant after taking antibiotics, the pregnancy rate was within the normal "typical use" failure rate for birth control pills. This suggests that there is not an increased risk of birth control failure while taking an antibiotic.
While clinical studies have not shown a clear interaction, there have been case reports of individuals accidently becoming pregnant while taking an antibiotic. Unfortunately, there is no way of predicting who (if anyone) is at an increased risk of becoming pregnant while taking an antibiotic.

Can Pregnancy Be Avoided While Taking Antibiotics?

It is still uncertain how significant the interaction between oral contraceptives and most antibiotics actually is (or even if it exists). Clinical studies suggest that there is not an increased risk of becoming pregnant when taking an antibiotic while on birth control. However, case reports of accidental pregnancies suggest that it may be a potential problem for some women taking the two medications together.
Backup methods of contraception, such as condoms, should be used if a woman on birth control also takes rifampin or griseofulvin, as there is a known interaction between these medications. Additionally, in situations where it is very important that a woman does not become pregnant, non-hormonal backup methods of contraception should be used when taking any antibiotic.
As always, it is best to discuss any concerns you may have about your treatment with your healthcare provider.
Last reviewed by: Kristi Monson, PharmD
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