Antibiotics may make the contraceptive pill less effective, scientists warn

Taking antibiotics may reduce the effectiveness of birth-control pills, researchers say.

Scientists are advising women to take extra precautions to avoid unwanted pregnancy when they are on the drugs that kill bacteria.

In a study published in the British Medical Journal’s Evidence-Based Medicine, they wrote: “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.”

But experts say most contraceptive pills in the UK come with warnings about possible reduced effectiveness when also taking some other drugs, including antibiotics.

Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine, who was not involved in the study, said: “It seems likely that the patient leaflets are not read, and it is possible that some prescribers do not mention the problem, or that patients forget they were told.”

The scientists, from the Nuffield Department of Primary Care Health Sciences at Oxford University, looked at data gathered through the Yellow Card scheme run by regulators, and said their findings suggested unintended pregnancies were seven times more common in women on antibiotics.

Accidental pregnancies were 13 times more common in women taking enzyme-inducing drugs – medicines known to impair the effectiveness of hormonal contraceptive – compared with other medicines.

But Prof Evans said that these reports “can almost never be taken as anything other than suggestions for causal effects”.

He said: “The problem is that these data neither provide good evidence that the disproportionate reporting is a causal effect, nor do they indicate the magnitude of the potential problem.

“With 46 unintended pregnancies reported over 55 years, even with under-reporting, it may not be a serious public health problem.”