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.