It usually begins after several months of treatment and may be irreversible. There have been no confirmed cases of TD directly caused by clozapine alone. Clozapine is effective for about 60 percent of those who try it. A patient should try clozapine for at least four to six weeks. Additional improvements may be noticed over six to twelve months.
Clozapine poses a unique risk.
Consequently, the FDA would not have approved it unless its effectiveness was proven clearly superior to that of current antipsychotic drugs. This was done conclusively in a 16-center study involving over 300 severely ill patients. These patients had been ill for many years and had failed to respond to at least three potent drugs. This improvement in psycho-social functioning was largely due to clozapine response. One to two percent of patients who take clozapine will develop a condition called agranulocytosis , in which the white blood cell count drops dramatically.