OBJECTIVE: This study sought to determine if amantadine affects weight gain in psychiatric patients taking olanzapine. METHOD: Twenty-one adults who had gained at least 5 lb with olanzapine were randomly assigned to receive amantadine (N=12) or placebo (N=9) in addition to olanzapine. The length of time taking olanzapine ranged from 1 to 44 months. Body mass index, psychiatric status, and fasting blood levels were assessed at baseline and 12 weeks. RESULTS: Significantly fewer subjects taking amantadine gained weight, with a mean change in body mass index of –0.07 kg/m2 for the amantadine group and 1.24 kg/m2 for the placebo group. This effect remained significant when the authors controlled for baseline body mass index and length of olanzapine treatment. No changes in fasting glucose, insulin, leptin, prolactin, and lipid levels were seen. Positive and Negative Syndrome Scale scores remained stable. CONCLUSIONS: Amantadine induced weight stabilization in subjects taking olanzapine and was well tolerated.