Women who take antibiotics while on the contraceptive pill are at risk of unintended pregnancy, a study suggests.
Experts have advised women who are prescribed antibiotics to use an additional form of contraception, such as condoms, as a precaution.
Researchers found that the drugs appear to reduce the effectiveness of hormonal contraceptives.
Experts have advised women who are prescribed antibiotics to use an additional form of contraception, such as condoms, as a precaution. Researchers found that the drugs appear to reduce the effectiveness of hormonal contraceptives (stock)
They looked through a national database showing the recorded side effects of medications, finding there were seven times as many unintended pregnancies reported for antibiotics than other drugs.
There have been suspicions that antibiotics could interfere with contraceptives since the early 70s, but the evidence is not conclusive.
The new study, published in the British Medical Journal, analysed unwanted drug side effects called ‘Yellow Cards’ which are reported to the Medicines and Healthcare products Regulatory Agency (MHRA).
These included 70,000 Yellow Cards for general antibiotics, 30,000 for enzyme-inducing antibiotics, and 65,000 for other types of drugs.
Overall, accidental pregnancies were seven times more common among people taking general antibiotics.
The enzyme-inducing antibiotics rifampicin and rifabutin, which can be used to treat tuberculosis and meningitis, come with an even greatest risk.
Unintended pregnancies were 13 times more common in women taking these drugs, which prompt the body to produce certain enzymes that interfere with hormonal contraceptives.
The new study, published in the British Medical Journal, analysed unwanted drug side effects called ‘Yellow Cards’. Discovered that accidental pregnancies were seven times more common among people taking general antibiotics (stock)
The authors, from the University of Oxford and University of Birmingham, wrote: ‘There were six unintended pregnancies in the Yellow Card reports of other drugs, 46 in the antibiotic reports, and 39 in the enzyme inducing drug reports.’
The researchers said it is impossible to calculate the overall risk and that it will vary from woman to woman.
But they concluded: ‘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.
‘Extra precautions can be taken during a course of antibiotics; an unintended pregnancy, whether terminated or taken to term, is a life-changing event.’
The NHS currently advises that women do not need to use additional contraception unless they’re taking the enzyme-inducing antibiotics rifampicin and rifabutin.
However, the patient information leaflet that comes with most types of antibiotics says they could affect your contraception.
Professor Stephen Evans, from the London School of Hygiene & Tropical Medicine, said: ‘Every packet of these relevant contraceptive pills has a patient leaflet in the UK and they contain warnings about possible reduced effectiveness when also taking some other drugs, including antibiotics. 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.
‘There are good reasons why antibiotics could reduce efficacy of some hormonal contraceptives, and the reminder may be helpful. 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.’