Lower plasma levels of haloperidol in smoking than in nonsmoking schizophrenic patients

The impact of smoking on plasma haloperidol (HAL) concentrations was invest igated in 66 Japanese male schizophrenic inpatients treated orally with HAL . The subjects consisted of 22 nonsmokers and 44 smokers each smoking ten c igarettes per day. Plasma concentrations of HAL were determined by an enzym e immunoassay method. There were significant positive correlations between the plasma HAL concentration and the daily dose of HAL per kg body weight ( Y = 58.1X - 0.01 (r = 0.86)). Smokers had significantly lower HAL concentra tions per daily dose of HAL/kg body weight than nonsmokers (smokers vs. non smokers = 54.3 +/- 16.6 vs. 70.6 +/- 23.2 ng/mL/mg/kg). In doses less than 0.2 mg/kg of HAL, smokers showed significantly lower HAL concentrations per daily dose of HAL/kg body weight than nonsmokers (smokers vs. nonsmokers = 55.1 +/- 14.4 vs. 79.5 +/- 27.1 ng/mL/mg/kg), whereas no significant diffe rence in HAL concentrations per daily dose of HAL/kg body weight was observ ed between smokers and nonsmokers when heated with more than 0.2 mg/kg (smo kers vs, nonsmokers = 52.9 +/- 20.7 vs. 60.0 +/- 11.1 ng/mL/mg/kg). Our res ults indicate that smoking may induce the enzyme(s) metabolizing HAL, which results in lower plasma HAL concentrations in smokers than in nonsmokers, particularly at low doses of HAL.