D’Souza et al. [44] conducted a randomized, double-blind, placebo-controlled study of Δ9-THC (0, 2.5, and 5 mg) effects in schizophrenic patients similar to the one described earlier in healthy subjects. The patients were taking stable doses of antipsychotic medications and were clinically stable. Δ9-THC transiently exacerbated a range of positive and negative symptoms, perceptual alterations, cognitive deficits, and medication side-effects associated with schizophrenia without producing any obvious “beneficial” effects. The increases in psychosis were brief, modest, and occurred even though subjects were clinically stable, medication-responsive and were receiving therapeutic doses of antipsychotics. The positive symptoms induced in these patients were similar to their typical symptoms. Using a threshold score of clinically significant positive symptoms (PANSS positive symptom subscale score ≥3 points) defined a priori, schizophrenia patients appeared to be more sensitive to Δ9-THC effects. Eighty percent of the schizophrenia group but only 35% of controls had a suprathreshold response to 2.5 mg Δ9-THC, and 75% of schizophrenic patients but only 50% of controls had a suprathreshold response to 5 mg Δ9-THC (Fig. 3). Similarly, relative to controls, schizophrenia patients were
