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.”
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 & 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.”
With 46 unintended pregnancies reported over 55 years, even with under-reporting, it may not be a serious public health problem
The scientists, led by Jeffrey K Aronson, of the Nuffield Department of Primary Care Health Sciences at Oxford University, looked at the data gathered through the Yellow Card Scheme run by the Medicines and Healthcare products Regulatory Agency (MHRA), the UK’s drug and medical devices regulator.
Among other things, the scheme collects information on unwanted side-effects of different types of drugs.
The researchers looked at more than 173,000 Yellow Card reports of unintended pregnancies in women taking antibiotics (74,623), enzyme-inducing drugs (32,872), and control medicines (65,578).
Enzyme-inducing drugs are medicines known to impair the effectiveness of hormonal contraceptives.
Results showed there were 46 unwanted pregnancies in the antibiotic Yellow Card 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 Yellow Card reports of antibiotics and 13 times more common in the reports of enzyme-inducing drugs, when 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.
“It would be good to have a proper observational study with a very much larger number of unintended pregnancies to elucidate the issue further.
“Overall this link is not novel and, as advice is already available in clinical practice, should be reported on with care as creating scares about oral contraceptives never has good outcomes.”
