Enzyme-inducing medicines: rifampicin, carbamazepine and St John's wort

The liver contains families of enzymes that chemically alter medicines so they can be removed. Some substances increase the amount of these enzymes. This is called induction. The consequence is faster clearance of any medicine that depends on those enzymes, which lowers its blood level and may reduce its benefit.

Induction differs from the grapefruit effect described in our other guide, which is inhibition and tends to raise levels. Inducers tend to lower them, so the typical risk is treatment failure rather than extra side effects.

Rifampicin is an antibiotic used mainly for tuberculosis and certain other serious infections. Carbamazepine is an epilepsy and nerve pain medicine. St John's wort is a herbal remedy taken for low mood that can be bought without a prescription. They are very different products with a shared effect on the liver.

Other medicines also induce enzymes, including phenytoin and phenobarbital, and the MedlinePlus tamoxifen page lists phenobarbital and rifampin among medicines to report to the prescriber. A pharmacist can say whether a given medicine belongs to the group.

NHS inform states that rifabutin and rifampicin can stop the combined pill working properly, and that epilepsy treatment and HIV medication also appear on the list. The NHS carbamazepine page says some types of hormonal contraception, such as the pill, patch or implant, may not mix well with it.

If you need a medicine of this kind, additional or alternative contraception should be discussed before starting. The same thinking applies to other methods, so ask the prescriber, GP or sexual health clinic about the method you use.

The NHS warfarin page says to avoid St John's wort entirely. The carbamazepine page says it interacts with warfarin and some other medicines to prevent blood clots, and the NHS rivaroxaban and apixaban pages list St John's wort among the herbal products that can cause problems. A loss of anticoagulant effect increases the risk of a clot or stroke.

Warfarin users have INR checks that can detect a change, but newer anticoagulants are not monitored in the same way, which is a reason for prescribers to avoid inducer combinations. See our guides on warfarin and on DOACs for the wider interaction patterns.

• Warfarin: avoid St John's wort; check any new inducer with the clinic.

• Apixaban and rivaroxaban: the NHS lists St John's wort as a problem.

• Never stop an anticoagulant on your own because of an interaction.

The NHS amlodipine page lists rifampicin and St John's wort among the things that may not mix well, and the atorvastatin page names St John's wort. The NHS omeprazole page names St John's wort too, and the levothyroxine page lists carbamazepine and St John's wort.

The list shows how many treatments cross paths with inducers, which is why a single herbal tablet can matter across a long list of prescriptions.

When an inducer is stopped, the enzymes gradually return to normal over days to weeks, and partner medicines can then build up. A dose that was raised to compensate during induction may become too much. Anyone stopping carbamazepine, rifampicin or St John's wort should tell the prescriber so doses can be reviewed.

Do not stop an epilepsy medicine or a course of TB treatment without medical advice; seek help urgently if you have a seizure or other worrying symptoms.