FSRH CEU Response to study: Analysis of reports of unintended pregnancies associated with the combined use of non-enzyme inducing antibiotics and hormonal contraceptives

Posted 19 August 2020

Date: 19 Aug 2020

Type: FSRH Clinical Guidance and Clinical Statements

The BMJ Evidence Based Medicine Journal has published a paper suggesting that antibiotics may lessen the effectiveness of hormonal contraception.

The authors1 used the ‘Yellow Cards’ system where clinicians and patients can report adverse drug side-effects to the UK’s drug and medical devices regulator, the Medicines and Healthcare products Regulatory Agency (MHRA). Data between 1963 and July 2018 was analysed and researchers compared the number of unintended pregnancies reported in 74,623 Yellow Cards for antibiotics in general and in 32,872 for enzyme-inducing drugs with those reported in 65,578 other types of drugs in users of oral contraception. There were 6 unintended pregnancies in the Yellow Card reports of other drugs, equivalent to 9/100,000 of the population; 46 in the antibiotic reports (62/100,000); and 39 in the enzyme inducing drug reports (119/100,000).

Compared with the other types of drug, unintended pregnancies were 7 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 7 times more often in enzyme-inducing drug Yellow Cards.

The authors conclude that ‘women taking hormonal contraceptives should be warned that antibiotics may impair their effectiveness. Extra precautions can be taken during a course of antibiotics’.

This new evidence is being discussed and a further statement will be released in the near future. Before there is a change to clinical guidance the limitations of this study need to be assessed. The most important of these is reporting bias. Clinicians and patients are more likely to report an unintended pregnancy to the MHRA as they believe that antibiotics interact with oral contraceptives.

After analysing all previously published studies the US Centers for Disease Control and Prevention (CDC)2 And the Clinical Effectiveness Unit (CEU) of the Faculty of Sexual & Reproductive Healthcare3 have stated that most broad-spectrum antibiotics do not reduce the contraceptive effectiveness of combined oral contraceptives, patches, or rings and extra precautions are not required when antibiotics are prescribed.


  1. 1. JK Aronson, RE Ferner. Analysis of reports of unintended pregnancies associated with the combined use of non-enzymeinducing antibiotics and hormonal contraceptives. BMJ Evidence-Based Medicine Published Online First: 18 August 2020. Available online here (accessed 19/08/2020)
  2. 2. Centers for Disease Control and Prevention. The United States Medical Eligibility Criteria for Contraceptive Use (USMEC). 2016. Available online here (accessed 19/08/2020)
  3. 3. Faculty of Sexual & Reproductive Healthcare (FSRH). Drug Interactions with Hormonal Contraception. January 2017, last reviewed 2019. Available online here (accessed 19/08/2020)


Read the full response to the study below or via this link.