Antibiotics and the contraceptive pill: what really reduces protection

For many years leaflets and conversations repeated that any antibiotic could make the pill fail. The belief is understandable: antibiotics can cause diarrhoea and sickness, and a few of them do change how the body handles hormones. The result was a blanket rule that obscured which cases mattered.

The current NHS wording separates two different mechanisms. One is an interaction in which the antibiotic changes how hormones are processed. The other is an absorption problem caused by illness, which can affect protection whatever the cause of the sickness or diarrhoea.

NHS inform says some antibiotics, such as rifabutin or rifampicin, can stop the combined pill working properly. These are used mostly for tuberculosis and some other serious infections, so most people will never be prescribed them.

If you are, additional contraception will be needed, and a prescriber should talk through alternative methods with you. Our enzyme-inducing medicines guide explains the mechanism shared with some epilepsy medicines and St John's wort.

The NHS amoxicillin common-questions page states that amoxicillin itself does not interfere with the pill but that if it makes you vomit, or causes severe watery diarrhoea or diarrhoea lasting more than 24 hours, your pills may not protect you from pregnancy.

NHS inform adds that if you vomit within 2 hours of taking the pill you should take another straight away and the next pill at the usual time, that very severe diarrhoea may mean the pill does not work properly, and that you should use extra contraception while sick and for 2 days after recovery.

• Vomiting within 2 hours: take another pill straight away.

• Very severe diarrhoea: keep taking pills but use additional protection.

• Continue extra precautions until 2 days after symptoms stop.

• Check your own pill packet for the pill-specific instructions.

Antibiotics are only one part of the list. The NHS carbamazepine page says some types of hormonal contraception, such as the pill, patch or implant, may not mix well with the epilepsy medicine. NHS inform lists HIV medication, epilepsy treatment and the diabetes and weight-loss medicine tirzepatide.

The NHS herbal medicines page also names hormonal contraceptives among prescribed medicines that herbal products may interfere with, so a herbal remedy bought for mood or sleep should be checked with a pharmacist.

Tell the prescriber or pharmacist which contraceptive you use. Ask whether the antibiotic is one that needs extra protection, and what to do if you are sick. Keep taking your pill unless advised otherwise; stopping it creates its own risk.

If a pill has been missed or may have failed and you have had unprotected sex, emergency contraception is time-sensitive; the NHS says it should be used within 3 to 5 days. A pharmacist or sexual health clinic can advise on the right option on the day.

Long-acting methods such as an intrauterine device do not depend on swallowing and absorbing a tablet, although other enzyme-inducing medicines can affect hormonal implants as the carbamazepine page indicates. Anyone starting long-term medicine that interacts can discuss switching methods with their GP or sexual health service.