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.