Enzyme induction and inhibition: how herbs, food and smoking change medicine levels

Liver and gut enzymes are not fixed machinery. The body can make more of them when exposed to certain chemicals, which is induction, or a substance can occupy or disable the enzyme so it cannot work on its usual targets, which is inhibition. Which one applies depends on the product, not on whether it is a prescribed medicine, a supplement or a food.

The outcome for another medicine depends on what the enzyme does to it. If the enzyme clears the medicine, induction lowers its level and inhibition raises it. If the enzyme activates the medicine, the pattern reverses. A pharmacist considers both the product and the other medicine's pathway.

Inhibition tends to start as soon as the blocking substance reaches the enzyme, and fades once it has been cleared. Induction involves the body producing more enzyme, so it takes time to build and time to unwind. This is why the interaction can outlast the product that caused it.

The MHRA's emergency-contraception notice illustrates the point: elevated CYP3A4 can persist for up to four weeks after stopping an enzyme-inducing medicine, and the advice for people who have used inducers in the past four weeks is to use a copper intrauterine device where possible. For smoking, the MHRA gives about a week for enzyme activity to recover after stopping.

St John's wort is sold without prescription and is widely seen as natural and mild. The MHRA's update says it has induction and inhibitory effects on several cytochrome P450 enzymes, and possibly acts through other routes as well. A case report described worsened seizure frequency and severity in a patient on levetiracetam, lamotrigine and clobazam, which were not drugs previously known to interact, so warnings were extended to all antiepileptic medicines.

The MHRA's advice is that people with epilepsy should not use products containing St John's wort. The NHS clopidogrel page also lists it as raising clopidogrel levels, and the MHRA's emergency-contraception update names it among CYP3A4 inducers. The theme is clear: treat herbal products as active ingredients and tell your pharmacist before starting one.

• Check labels for St John's wort in teas, capsules and mood-support products.

• Do not start or stop a herbal product alongside epilepsy, transplant, HIV or contraceptive medicines without advice.

• Remember that stopping an inducer is not instant; the interaction may last weeks.

The NHS simvastatin page advises avoiding grapefruit juice because it can stop the medicine working properly, and the MHRA says simvastatin users should avoid it and atorvastatin users should limit intake to very small amounts or avoid it. The NHS carbamazepine and warfarin pages also advise avoiding grapefruit juice.

These warnings do not mean every food is a problem. The NHS warfarin page makes a different point: foods high in vitamin K can affect that medicine through a clotting mechanism, not an enzyme one, and consistent intake is the aim. For most medicines no food restriction applies, and the patient leaflet states what to avoid.

Tobacco smoke induces CYP1A2 through combustion chemicals, which can lower levels of theophylline, clozapine, olanzapine and other medicines, and leave levels rising after quitting. The MHRA recommends reviewing doses when smoking status changes.

Alcohol can also interact with many medicines, including simvastatin, where the NHS says drinking too much can increase side effects. Both examples show why a medicines review should include lifestyle as well as prescriptions.

Keep an up-to-date list of everything you take, including over-the-counter products, supplements and herbal remedies, and show it at every review. Use one pharmacy where possible so interactions can be checked consistently.

If you notice new symptoms after starting a product, such as seizures returning, unexpected bleeding, severe muscle pain or loss of contraceptive effectiveness, seek advice promptly. For emergencies such as a seizure that does not stop, collapse or heavy bleeding, call 999. Suspected interactions can also be reported through the MHRA Yellow Card scheme.