Relationships between psychotropic drug dosage, plasma drug concentration, and prolactin levels in nursing home residents.

This study aimed to characterize the relationships between administered dosages of psychotropic drugs, plasma drug concentration, and prolactin levels in a group of elderly nursing home residents. In a randomized, placebo-controlled, double-blind crossover design study, blood samples were drawn from 47 nursing home residents at least 6 hours after taking either haloperidol, thioridazine, or lorazepam. Correlations between drug dosage and plasma drug levels were significant for haloperidol and thioridazine, but not for lorazepam. Plasma drug levels were below the levels of detection for most of those taking haloperidol. Lorazepam was detected in the blood of 4 of the participants even after 3 weeks of downward titration to placebo and 6 weeks of placebo. Prolactin level was related to administered dosage only in those who were taking haloperidol. For those taking haloperidol or thioridazine, prolactin levels decreased when participants were on placebo. When an older person is taken off lorazepam, the possibility of residual drug in their bodies even 6 weeks after termination of drug use should be considered. Haloperidol may be clinically active in the brain despite no currently detectable plasma drug concentration.