Smokers have a significantly higher risk of experiencing adverse drug reactions compared with non-smokers. This increased risk is largely due to the interaction between the more than 4,000 substances found in tobacco smoke and the medications being taken.
Tobacco smoking can alter both the effectiveness and tolerability of many medicines by affecting their pharmacodynamic and pharmacokinetic properties.
Drug-smoking interactions
As explained in an article published by Focus Farmacovigilanza drug-smoking interactions are primarily caused by the effects of tobacco smoke on the activity of cytochrome P450 metabolic enzymes.
The best-known effect of smoking is the induction of the CYP1A2 isoenzyme, which increases the metabolism of drugs processed by this enzyme. As a result, plasma concentrations of the active substance may decrease, potentially reducing the medication’s therapeutic effect.
One example is the antipsychotic drug clozapine. Smoking between 7 and 12 cigarettes per day is sufficient to achieve maximum enzyme induction and may require a 50% increase in the clozapine dose to maintain adequate plasma concentrations and clinical efficacy.
Conversely, when a patient stops smoking, CYP1A2 activity gradually returns to near-normal levels. Therefore, in individuals receiving antipsychotic treatment, medication doses may need to be adjusted following smoking cessation to avoid excessive drug exposure.
Similar situations can occur with other medications, including treatments used for cardiovascular diseases. According to data published in a recent meta-analysis, the dosage of warfarin may need to be approximately 12% higher in smokers than in non-smokers.
Understanding these interactions is important for ensuring safe and effective treatment, highlighting yet another reason why smoking cessation can have significant health benefits.