Stopping smoking effects on drug metabolism

It is widely known that smoking speeds up basal metabolic rate (by around 10%) and so stopping smoking slows down the basal metabolic rate. This is one of the reasons people tend to put on weight when they stop smoking (the other reason is increased appetite resulting from the loss of nicotine’s mild appetite suppressant effects).

However, it is less widely known that cigarette smoking increases the activity of several liver enzymes that are responsible for metabolizing a number of drugs/medicines. This means that smokers frequently require a larger dose of these medicines than non-smokers, and may require the dose to be reduced after they quit smoking. One of the drugs whose metabolism is affected by smoking is caffeine. So when someone who drinks 6 cups of caffeinated coffee per day stops smoking, those 6 cups will produce blood caffeine levels up to 50% higher and so they will therefore feel as if they had about 9 cups of coffee. In the case of caffeine this could cause the person to feel anxious, restless and jittery and may also make it harder to get to sleep.

Many of the medicines that are affected by smoking are medicines used to treat psychiatric problems. These include some members a class of antianxiety drugs known as, “benzodiazepines” (e.g. diazepam), a number of anti-psychotic medicines (including some of the newer “atypical” anti-psychotic medicines e.g. olanzapine (Zyprexa), as well as older ones like haloperidol [Haldol]). Metabolism of some antidepressants is also affected in a similar way as are some other (non psychiatric) medicines. So, as with caffeine, when people who are already taking these medicines on a regular basis then quit smoking, the blood levels of these medicines may well increase by 10-40%. So depending on the medicine, this may also cause an increase in some of the side-effects caused by the medicine.

All of this serves to underline that it is a good idea to speak to your family doctor around the time you plan to quit. The family doctor should be able to identify the medicines whose metabolism is affected by tobacco, and also identify the likelihood that any new side effects that begin after ceasing smoking may be due to the effects on drug metabolism.

A list of drugs whose metabolism is known to be affected by smoking (and therefore smoking cessation) may be found at:
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About the Author

MA, MAppSci, PhD

Dr. Jonathan Foulds is an expert in the field of tobacco addiction.