PROLONGED HALOPERIDOL AND REDUCED HALOPERIDOL PLASMA-CONCENTRATIONS AFTER DECANOATE WITHDRAWAL

Haloperidol and reduced haloperidol plasma concentrations were measured in twelve schizophrenic patients upon cessation of haloperidol decanoate (HLD) treatment. Each patient received HLD 100 mg every 4 weeks for five injections. After the fifth injection, HLD was discontinued. Haloperidol and reduced haloperidol plasma concentrations were obtained prior to cessation and at weeks 1, 3, 4, 5, 7, 9, 11, and 13 post-injection. Haloperidol and reduced haloperidol plasma concentrations were assayed by HPLC. Both haloperidol and reduced haloperidol plasma concentrations were detectable 13 weeks post HLD discontinuation. Maximal haloperidol plasma concentrations were observed at one week post cessation and gradually declined. The mean elimination half-life for haloperidol was 27.4 +/- 8.6 days (range 19.0-47.0 days). Reduced haloperidol plasma concentrations declined very slowly. Our results show that both haloperidol and reduced haloperidol plasma concentrations can remain for extended time periods after HLD is discontinued.