Scientists at Oxford University have advised that women should take extra precautions to avoid unwanted pregnancy if they are taking antibiotics, after a study found the medication can stop pills from working.
The new study, published in the British Medical Journal’s Evidence Based Medicine, analysed data gathered through the Yellow Card Scheme, which is run by the Medicines and Healthcare products Regulatory Agency (MHRA), the UK’s drug and medical devices regulator. The scheme collects information on unwanted side effects of different types of drugs.
The researchers assessed more than 173,000 Yellow Card reports, 74,623 of which included women taking antibiotics.
Findings showed that of those on antibiotics, 46 women reported unwanted pregnancies, suggesting that the drugs can impair the function of hormonal contraceptives. This is equivalent to 62 in 100,000 of the population.
The findings showed that 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.
In the report, scientists said, “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 drug is added.”
Despite the new findings, experts have said that most contraceptive pills in the UK already come with warnings about possible reduced effectiveness when taking some other drugs, including antibiotics.
The possible lack of awareness could stem from patients not fully reading the information leaflets, or prescribers failing to mention the problem when issuing the medication.
Stephen Evans, a professor of Pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, who was not involved in the study, warned that further research is needed to determine the effects of antibiotics on hormonal contraception.
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.”