Fluctuation of serum zuclopenthixol concentrations in patients treated with zuclopenthixol decanoate in viscoleo.

Zuclopenthixol serum concentrations were measured in 58 psychiatric patients referred for routine therapeutic drug monitoring (TDM). Patients were treated for prolonged time with zuclopenthixol decanoate in viscoleo in doses of 50-500 mg, administered intramuscularly at 14-day intervals. The serum concentration was determined at days 7 (C7) and 14 (C14) following injection. The mean ratio C7/C14 was 2.0 and was independent of the dosage given. In 14 patients, additional blood samples were drawn at day 3 (C3) following injection. The mean ratio C3/C14 of this group was 3.2. An almost log-linear decline of the serum concentration from day 3 to 14 appeared, which corresponds to an apparent half-life of zuclopenthixol in this dosage form of 7.4 days. The marked fluctuations of serum concentrations of zuclopenthixol from peak to trough levels in patients given fortnightly injections of the depot preparation indicate that shorter intervals between injections should be considered in many cases in order to diminish side effects.