New research linking commonly prescribed antibiotics with reduced effectiveness of hormonal contraceptives is misleading

FAMILY Planning NSW is concerned new research suggesting all antibiotics may reduce the effectiveness of hormonal contraceptives could cause unnecessary anxiety among women using hormonal contraception and their prescribing doctors.

Family Planning NSW Medical Director Dr Deborah Bateson said women should have confidence in the effectiveness of their hormonal contraceptive method if they are using it correctly, and be assured that the leap this new study makes to question the effectiveness of all methods of hormonal contraception with any antibiotic use is hard to uphold.

The study, published in BMJ Evidence-Based Medicine, analyses data collected between 1963 and 2018 from databases in the UK to link unintended pregnancies with the use of three groups of medications: enzyme inducing medications (including some anti-epileptics, antiretrovirals used for HIV, rarely used antibiotics called rifampicin and rifamycin used for tuberculosis, and some over the counter preparations such as St John's Wort) commonly prescribed antibiotics such as amoxycillin, cephalexin and erythromycin, and a control group of other medications. The enzyme inducing medications are already known to reduce the effectiveness of some hormonal contraception.

The analysis does not include actual use of hormonal contraception but is based on the assumption that all reported unintended pregnancies in women who were taking one of the medications included in the study were also using hormonal contraception at the time.

"There are many possible biases in the reporting of the data, and studies such as this can only look at associations and cannot prove that unintended pregnancies are caused by taking particular medications" says Dr Bateson.

The study found there were 46 unintended pregnancies in the antibiotic reports which is equivalent to 62/100,000 women, 39 in the enzyme inducing medication reports (119/100,000) and 6 in the other medication reports (9/100,000 women).

"As the data spans 55 years, the reports of only 46 pregnancies for the antibiotic group is very reassuring. The WHO and other professional bodies concluded in 2009 and again in 2015 that broad spectrum antibiotics do not reduce the effectiveness of hormonal contraception and advised that additional precautions were not necessary. This study does not change this advice," Dr Bateson said.

People using enzyme inducing medications (including antiepileptics, antiretrovirals and rifampicin and rifamycin) are advised to switch from a contraceptive pill, vaginal ring and the contraceptive implant to a method such as a hormonal or copper intrauterine device (IUD) or a contraceptive injection where the effectiveness is not reduced by these medications. Alternatively, doctors may advise taking additional precautions for the course of the treatment and the 28 days after it is stopped. People using commonly prescribed antibiotics do not require additional precautions with their hormonal method of contraception.

If anyone has concerns or wants to discuss their contraceptive options, they should speak to their doctor, visit their nearest family planning clinic or call the Family Planning NSW Talkline.

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