Smoking cigarettes induces primarily the cytochrome P450 1A2 isoenzyme, which more rapidly metabolizes many of the antipsychotic medications. Most notably, the second generation antipsychotics impacted include olanzapine, clozapine, and asenapine. The first generation antipsychotics impacted include chlorpromazine and to some extent fluphenazine, perphenazine, and haloperidol. When a patient is taking one of these antipsychotic medications and smoking cigarettes moderately to heavily, the medication will be quickly metabolized from the circulation and be overall less effective, requiring higher doses for symptom control and possibly creating more side effects.