Stopping smoking and changes in medicine levels

Tobacco smoke contains many substances that switch on liver enzymes. For medicines cleared by those enzymes, smokers get through the dose faster, which can mean they need more of it to get the same effect. When smoking stops, the enzymes return to normal over days to weeks, the medicine is cleared more slowly and the blood level climbs.

This is an effect of the smoke, not of nicotine. That is why using nicotine replacement does not prevent it, and why the NHS varenicline page says dose changes may be needed when you stop smoking whether or not you use varenicline.

The NHS lists theophylline and aminophylline for breathing problems, erlotinib for cancer, riociguat for lung hypertension, warfarin for clot prevention, insulin for diabetes and olanzapine and clozapine for psychosis. The list is illustrative; the prescriber will know of others.

For each, the effect of stopping smoking differs. A rise in theophylline or clozapine can bring side effects. Warfarin control may shift, so the INR needs checking. Insulin needs may change, so blood sugar monitoring matters. Prescribers decide the new dose; you should not adjust it yourself.

• Theophylline and aminophylline.

• Warfarin.

• Insulin.

• Olanzapine and clozapine.

• Erlotinib and riociguat.

ASH explains that tobacco smoke interacts with some psychiatric medication, making it less effective and resulting in higher doses, and that when a person stops smoking, their dose can often be reduced. It gives clozapine as an example and notes that monitoring and adjustment are needed after a quit attempt.

People in mental health services, including inpatient wards that have gone smoke-free, may therefore have dose changes as part of stopping. This should be planned by the team, not left to chance.

Before the quit date, tell your GP, pharmacist or the stop smoking adviser which medicines you take. Ask whether a blood test, INR check or blood sugar plan is needed in the following weeks, and who to contact if you feel different. Keep appointments for monitoring even if you feel well.

Our fetched NHS pages do not give a precise timetable for each medicine, so rely on the plan agreed with your own prescriber rather than a general rule.

The effect works in reverse. If you relapse, enzymes are switched back on and levels can fall, so a dose reduced after quitting may become too low. Tell the prescriber if you restart or cut down significantly, including when you switch to vaping or nicotine products, since the picture can change.

The NHS page does not cover every product, so the safest approach is to report any change in smoking pattern when you are on a medicine with a narrow safety margin.

Report new drowsiness, tremor, nausea or palpitations to the prescriber promptly. With warfarin, report unusual bruising or bleeding. With insulin, report frequent low blood sugars. Seek urgent help via NHS 111, or 999 if severe, for any emergency symptom.

Stopping smoking remains worthwhile, and these adjustments are usually routine for the team managing them.