Clozapine effectiveness in the presence of other meds

My wife has been on Trilafon and recently added Clozaril for her Schizphrenia. She is still having problems. Her Dr. says that other anti-psychotics can interfere with clozaril's effectiveness. Is this true, has any evidence been found to support this?

Thankx,
Dwight

[HFHS MD - RG]

The information packet of Clozaril indicates that the risks of  using Clozaril in combination with other drugs have not been systematically evaluated.

However, the concomitant use of Clozaril with other drugs (like Trilafon)metabolized by a liver enzyme named cytochrome 450 2D6 should be approached with caution because these individuals may develop higher concentrations of Clozaril in the blood that may cause an increase incidence of side effects from the Clozaril.