Antibiotics 'may reduce effectiveness of contraceptives'

The study found unintended pregnancies were seven times more common in reports where antibiotics were also used than when compared with other medicines. File Picture.

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

In a study, published in the British Medical Journal's Evidence Based Medicine, scientists advise women should take extra precautions to avoid unwanted pregnancy when they are on antibiotics.

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."

The scientists, led by Jeffrey K Aronson, of the Primary Care Health Sciences at Oxford University, looked at the data gathered from the UK's drug and medical devices regulator.

Among other things, the data included information on unwanted side-effects of different types of drugs.

The researchers looked at more than 173,000 reports of unintended pregnancies in women taking antibiotics (74,623), enzyme-inducing drugs (32,872), and control medicines (65,578).

Results showed there were 46 unwanted pregnancies in the antibiotic reports, equivalent to 62 in 100,000 of the population.

There were also six unintended pregnancies in the reports of control drugs (9/100,000) and 39 in the reports of enzyme-inducing drugs (119/100,000).

According to the researchers, the findings suggest unintended pregnancies were seven times more common in the reports of antibiotics and 13 times more common in the reports of enzyme-inducing drugs, when compared with other medicines.

But Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, who was not involved in the study, said 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.