Researchers have found evidence to support anecdotal reports of antibiotics reducing the effectiveness of hormonal contraceptives, it was announced today.
“Suspicion that antibiotics might reduce the efficacy of oral contraceptives was first raised in 1973,” say Dr Robin Ferner of the University of Birmingham, UK, in today’s (19 August) BMJ Evidence Based Medicine.
However, formal studies have either suggested that there is an interaction of this kind but without conclusive evidence – or have shown no evidence of an interaction.
The researchers write: “Most previously negative or inconclusive studies have been too small to rule out an interaction in a small subset of women, and other criticisms of previous studies leave the question of an interaction open. An interaction of this kind would be of great clinical importance.”
They looked at the rate of unintended pregnancies in women exposed to antibiotics by reviewing ‘Yellow Card’ reports of suspected unwanted drug side effects sent to the UK’s Medicines and Healthcare products Regulatory Agency.
This showed that unintended pregnancies were seven times more commonly reported with antibiotics than with other medicines. Unintended pregnancies were also 13 times more common in reports of enzyme-inducing drugs, which included some antibiotics.
The authors conclude: “This evidence suggests there is an interaction of antibacterial drugs with hormonal contraceptives, which can potentially impair the effectiveness of the contraceptives.
“Women taking hormonal contraceptives should be warned that antibiotics may impair their effectiveness. Extra precautions can be taken during a course of antibiotics; an unintended pregnancy is a life-changing event.”
Aronson, J. K. Analysis of reports of unintended pregnancies associated with the combined use of non-enzyme-inducing antibiotics and hormonal contraceptives. BMJ Evidence Based Medicine 19 August 2020; doi: 10.1136/bmjebm-2020-111363
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