Clozapine effects on force control in schizophrenic patients.

We previously reported significant differences in force control (FC) function between schizophrenics treated with typical antipsychotic drugs (APD) and those treated with clozapine. Clozapine treatment was associated with an attenuation of the capacity for fine motor control. We now report that a test-retest study with 41 treatment-refractory patients confirms our earlier finding; the FC deficit is due primarily to clozapine treatment. An additional comparison was made with 10 patients who were administered the FC test repeatedly through the initial clozapine titration interval of 6-8 weeks. The results suggest that two distinct clozapine effects can be distinguished, an initial transient stage characterized by 'drowsiness' and a subsequent stage with dose-dependent emerging myoclonic features.