Weight gain is associated with treatment with olanzapine and other psychotropic agents. Nizatidine, a histamine H-2 receptor antagonist, has been proposed to have weight-reducing effects. This double-blind trial evaluated the efficacy of nizatidine in limiting weight gain in patients with schizophrenia and related disorders who were treated with olanzapine for up to 16 weeks. After an initial screening period, 175 patients were randomized to receive olanzapine (5–20 mg) with either placebo or nizatidine (150 mg b.i.d. or 300 mg b.i.d.). Significantly less weight gain was observed on average at weeks 3 and 4 with olanzapine+nizatidine 300 mg b.i.d. (P<0.05) compared to olanzapine+placebo, but the difference was not statistically significant at 16 weeks. Nizatidine was well-tolerated and did not adversely affect clinical outcomes. Nizatidine 300 mg b.i.d. may have an early transient effect in limiting the weight gain, but this potential early effect appeared to be diminished or eliminated by 16 weeks.