¿Qué medicamentos pueden afectar tu anticoncepción?

Suspicions that antibióticos might reduce the effectiveness of hormonal contraceptives date back to the 1970s but conclusive evidence on the subject has been hard to find. Now, un estudio suggests women taking hormonal birth control should take extra precautions to avoid unintended pregnancy if they are prescribed antibiotics.

But this contradicts current advice which says most antibiotics don't interfere with the pill and other hormonal contraceptives like the patch, implant and ring. The exception is drugs used to treat serious bacterial infections such as tuberculosis and meningitis called 'rifampicin-like' antibiotics. They include rifampicina y rifabutina and there is evidence that these medications can stop hormonal contraception working as well as it should do.

Which medicines affect the pill?

There are a few other types of medications that you have to be careful about if you're taking hormonal contraception. Some medicines are 'enzyme-inducing', meaning they can speed up the way the body processes other medications (including contraceptive hormones and anticoncepción de emergencia), making them less effective.

Enzyme-inducing medicines include the rifampicin-like antibiotics, some drugs used to treat epilepsy, some HIV medicines and Hierba de San Juan, a herbal remedy for depresión.

Dr Catherine Armitage, spokesperson for the Faculty of Sexual and Reproductive Healthcare (FSRH), says doctors and nurses prescribing hormonal contraception should always ask women about their current and previous medicine use. And this should include "over-the-counter medicines, herbal remedies, recreational drugs and dietary supplements" as it's not just prescribed medications that can cause interactions.

Dr Shree Datta, gynaecologist and spokesperson for menstrual cup brand Intimina, points out that weight-loss medications such as orlistat might affect the contraceptive pill too. This is because if diarrea occurs as a side effect, it could decrease the absorption of the pill's hormones into the bloodstream; diarrhoea or vomiting due to any cause can have the same effect.

"It's important to make sure your doctor is aware of what medications you are currently taking so that we can consider whether your medications will interact and affect how they work," she suggests. "Bring a list or photo if you're unsure as it can be difficult to remember the drug names!"

In the antibiotics study, published in BMJ Evidence-Based Medicine, researchers used reports of suspected unwanted drug side effects, known as 'Yellow Cards' flagged up by doctors and the public to the UK's medicines regulator, the Medicines and Healthcare products Regulatory Agency (MHRA).

They looked at the number of unplanned pregnancies reported for three sets of medicines: enzyme-inducing drugs (like the ones mentioned above), non-enzyme-inducing antibiotics and medicines used by women of childbearing age which have no known or suspected interactions with hormonal contraception. "We picked these at random from a hospital formulary," reveals research co-author Professor Robin Ferner from the University of Birmingham.

Compared with medications that weren't thought to interact with hormonal contraception, unintended pregnancies were seven times more common in Yellow Card reports of antibiotics and 13 times more common in reports of enzyme-inducing drugs.

The research suggests women taking hormonal contraceptives who are prescribed antibiotics should take extra precautions to avoid pregnacy if appropriate, such as using condoms for seven days after finishing the course.

But according to the FSRH, which analysed all previously published studies on the subject, this might be unnecessary. It found most antibiotics don't reduce the effectiveness of hormonal contraception.

In a statement, the organisation said: "Before there is a change to clinical guidance the limitations of this study need to be assessed. The most important of these is reporting bias. Clinicians and patients are more likely to report an unintended pregnancy to the MHRA as they believe that antibiotics interact with oral contraceptives."

This means patients who took antibiotics are likely to believe the unplanned pregnancy occurred because of the medicine, but it may have happened for other reasons such as forgetting to take the pill on one or more days. There is a particular risk of becoming pregnant if you miss pills at the end or beginning of a packet. And even if taken perfectly, no type of contraception is 100% effective.

The guidelines around antibiotics and hormonal contraception may change in the future. Armitage advises women to speak with their doctor before changing or stopping any course of medication about whether it will affect their contraception.

"They will be able to advise the best course of treatment to make sure women can take their medications safely while using a suitable method of contraception to prevent unplanned pregnancies," she says. "If your practice has a clinical pharmacist working for them, they may be able to advise on this too."

There are safe and effective alternatives for people who aren't able to use hormonal contraception because of medication interactions. "The copper IUCD, one of the most effective methods, as well as the inyección anticonceptiva, are not affected by any drug interactions," says Armitage.

For women taking the contraceptive pill, Datta emphasises the importance of taking it at the same time each day and making sure to start a new pill packet on the date you're meant to. Read the leaflet that comes with your brand of pill carefully so you know what to do if you miss a day, as you may need to take extra precautions.

"We want to make sure both your contraceptive pill and any other medication can do the job they need to do," she says.