Switching to e-cigarettes affects drug concentration

Cytochrome P450 subtype 1A2 (CYP1A2) is responsible for the metabolism of several drugs, including the antipsychotic agent clozapine. Smoking cigarettes induces CYP1A2. Due to this induction, a higher dosage of the drug is required by patients who smoke tobacco.The dosage of clozapine was changed for a 34-year-old male because of suspected active psychosis. However, serum levels of clozapine did not change according to expectations. It appeared that the patient had switched from smoking normal cigarettes to using e-cigarettes and then started smoking cigarettes again during the time of the dosage change. This gave a plausible explanation for the observed clozapine concentrations.When patients switch from smoking normal cigarettes to using e-cigarettes induction of the CYP1A2 enzyme stops. A switch of this type may result in a major, undesirable increase in drug concentrations. Doctors should be alert to this when prescribing medication metabolised by CYP1A2 with a narrow therapeutic window.