Serum concentrations of risperidone and 9-OH risperidone following intramuscular injection of long-acting risperidone compared with oral risperidone medication.

OBJECTIVE: To compare serum concentrations of risperidone, 9-hydroxy (OH) risperidone and risperidone plus 9-OH risperidone, as well as the 9-OH risperidone/risperidone ratio in patients receiving depot and oral risperidone.
METHOD: Serum concentrations from 78 patients receiving three different doses of risperidone depot were measured and compared with serum concentrations from 82 patients taking three different doses of oral risperidone.
RESULTS: Patients receiving risperidone depot had significantly lower serum concentrations of risperidone plus 9-OH risperidone than patients taking oral risperidone. More interestingly, the 9-OH risperidone/risperidone ratio was also significantly lower in patients receiving risperidone depot than in patients taking oral risperidone.
CONCLUSION: Serum concentrations of risperidone plus 9-OH risperidone may be a rather poor indication of the antipsychotic efficacy of risperidone unless their ratio is also considered.