Antibiotics may lessen the effectiveness of hormonal contraceptives, finds an analysis of unwanted side effects associated with the combined use of these drugs, and published in BMJ Evidence Based Medicine.

Women on the Pill should be advised to take extra precautions to avoid unintended pregnancy, when prescribed antibiotics, advise the researchers.

Suspicions that antibiotics, particularly those that cover a wide range of bacteria, known as broad spectrum antibiotics, might reduce the effectiveness of hormonal contraceptives, date back to 1973. Since then several anecdotal reports have also implicated various antibiotics in weakening the effects of hormonal contraceptives.

Published studies have either suggested that antibiotics and hormonal contraceptives don’t mix, but haven’t provided conclusive evidence, or have concluded that there isn’t any evidence of interference.

Current advice, which is based on a few small studies, is that antibiotics, other than those that prompt the production of certain enzymes, don’t interfere with hormonal contraceptives.

To explore this further, the researchers drew on reports of suspected unwanted drug side effects, known as ‘Yellow Cards’, flagged up by clinicians and the public to the UK’s drug and medical devices regulator, the Medicines and Healthcare products Regulatory Agency (MHRA).

They 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.

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 researchers caution that it’s 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, so is highly unlikely to apply universally.But they conclude: “This evidence suggests there is an interaction of antibacterial drugs with hormonal contraceptives, which can potentially impair the effectiveness of the contraceptives.

“The precautionary principle dictates that women taking hormonal contraceptives should be advised to take extra contraceptive precautions when a short course of an antibacterial drugs is added.”