Women on the pill should be warned that taking antibiotics may lead to unwanted pregnancies, Oxford University study says

As it stands, the NHS says: “Most antibiotics do not affect contraception. It’s now thought that the only types of antibiotic that interact with hormonal contraception and make it less effective are rifampicin-like antibiotics.”

But several anecdotal reports have implicated various antibiotics in weakening the effects of hormonal contraceptives.

To explore this further, the scientists, led by Jeffrey K Aronson, of the Nuffield Department of Primary Care Health Sciences at Oxford University, drew on reports of suspected unwanted drug side effects, known as ‘Yellow Cards’, flagged up by clinicians and the public to the UK’s drug and medical devices regulator, the Medicines and Healthcare products Regulatory Agency (MHRA).

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).

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).

As the NHS says, enzyme-inducing drugs are already known to impair the effectiveness of hormonal contraceptives.

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.

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