Antibiotics May Lessen Effectiveness of Hormonal Contraception

Priscilla Lynch 

August 21, 2020

Antibiotics may lessen the effectiveness of hormonal contraceptives suggests an analysis of unintended pregnancies associated with the combined use of non-enzyme-inducing antibiotics and hormonal contraceptives published in  BMJ Evidence-Based Medicine.

While it is known that enzyme-inducing antibacterial drugs can impair the efficacy of hormonal contraceptives, there has long been suspicion that other antibiotics might do likewise despite a lack of definitive studies. Current advice that the purported interaction does not occur is based on small studies observing few pregnancies, assumes that all women are susceptible, and doesn't measure unbound hormone concentrations.

To test the null hypothesis that antibiotics do not impair the effectiveness of oral contraceptives, researchers conducted a database review of suspected adverse drug reactions, ‘Yellow Cards’, reported to the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) between 1963-July 2018.

They compared the number of unintended pregnancies associated with Yellow Cards reported in people taking antibacterial drugs (n=74,623), enzyme-inducing medicines (n=32,872), or control medicines (n=65,578).

There were 46 unintended pregnancies in the Yellow Card antibiotics reports (62/100,000); 39 in the enzyme-inducing drug reports (119/100,000), and six in the control drug reports, (9/100,000).

Compared with the other types of drug, unintended pregnancies were seven times more common in Yellow Card reports of antibiotics and 13 times more common in reports of enzyme-inducing drugs, which included some antibiotics. Congenital birth defects were also reported seven times more often in enzyme-inducing drug Yellow Cards.

Women taking the contraceptive pill should thus be advised to take extra precautions to avoid unintended pregnancy when prescribed antibiotics, advise the researchers.

However, they caution that it is impossible to calculate absolute risks from the data presented. The risk will also vary from woman to woman according to her physiological make-up and circumstances.

Aronson JK, Ferner RE. Analysis of reports of unintended pregnancies associated with the combined use of non-enzyme-inducing antibiotics and hormonal contraceptives. BMJ Evidenced-Based Medicine. 2020 August 18 [Epub ahead of print]. doi 10.1136/bmjebm-2020-111363.

This article originally appeared on Univadis, part of the Medscape Professional Network.

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