Common antibiotics such as amoxicillin and trimethoprim are linked with an increased risk of unintended pregnancies in women taking the oral contraceptive pill (OCP), researchers say.

Their analysis of adverse drug reactions covering 55 years found nine common non-enzyme-inducing antibiotics (see box) were linked to a seven-fold higher risk of unintended pregnancies compared with other medicines commonly used by women of child-bearing age.

The study also confirmed the accepted concern over enzyme-inducing antibiotics, including rifampicin, finding a 13-fold higher risk of unplanned pregnancy compared with the control medicines.

The researchers, from the Centre for Evidence-Based Medicine at Oxford University, in the UK, analysed more than 173,000 reports of suspected drug reactions made to the UK Medicines and Healthcare products Regulatory Agency (MHRA).