Antibiotics and Oral Contraceptives: Should We Worry?

Andrew M. Kaunitz, MD


September 18, 2020

This transcript has been edited for clarity.

Since oral contraceptives first became available, there have been concerns that antibiotics might interfere with their efficacy. However, a review of studies examining pharmacokinetic outcomes and suppression of ovulation by Centers for Disease Control and Prevention (CDC) authors concludes that common non-enzyme–inducing antibiotics do not impair the effectiveness of oral contraceptives.

In a recent report published in BMJ Evidence-Based Medicine (which has received media attention), investigators reviewed thousands of spontaneous reports of suspected adverse drug reactions (ADRs) submitted to Britain's regulatory authority. These ADRs referred to unintended pregnancy while taking medications in one of three categories:

  • Commonly used non-enzyme–inducing antibiotics, including ampicillin, cephalosporins, tetracyclines, and metronidazole

  • Hepatic enzyme-inducing medications known to interact with some hormonal contraceptives, including carbamazepine, nevirapine, and rifampin

  • Control medications, including citalopram, ibuprofen, and zolpidem, commonly used by reproductive-age women and not known to impact efficacy of hormonal contraceptives

ADRs involving control medications included nine pregnancies reported per 100,000 ADRs. The rates were 62 and 119, respectively, for ADRs involving non-enzyme–inducing antibiotics and enzyme-inducing medications.

The authors indicate that the sevenfold higher rate of reported unintended pregnancies with antibiotics "...constitutes a signal of possible drug-drug interaction."

Keep in mind, though, that this analysis is entirely based on spontaneous reporting of adverse events. As the authors acknowledge, such passive reporting probably reflects clinicians' biases. For instance, if clinicians suspect that antibiotics may cause oral contraceptive failure, they may be more likely to submit a report when a pregnancy occurs in an antibiotic user compared with a woman using one of the control medications.

The key cause of unintended pregnancy among women using short-acting hormonal contraception, including oral contraceptives, is inconsistent use, an observation which underscores the advantages of intrauterine, implantable, and injectable contraception.

In my practice I will continue to follow guidance from the CDC which states that antibiotics other than rifampin do not attenuate the efficacy of contraceptive pills, patches, or rings. In women chronically using known enzyme-inducing medications, IUDs and depot medroxyprogesterone acetate represent sound choices.

Thank you for the honor of your time. I am Andrew Kaunitz.

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