Phase 2: Subjects may enter Phase 2 following Phase 1 or enter directly if there is documentation of more than 1 but less than 3 BMI units of weight gain and they have started olanzapine within the previous 12 weeks. Phase 2 is a double blind placebo controlled addition of amantadine to ongoing olanzapine therapy. All subjects entering Phase 2 will have anthropometric measurements, and BMI will be calculated. Fasting lipid profile, glucose, insulin, C reactive protein, adiponectin, and leptin will be obtained, as will liver function tests and a urine drug screen. Body composition by DXA as well as RQ and REE will be determined. Prior to every DXA scan the Body Composition Lab will administer a safety questionnaire and women of reproductive age will have a pregnancy test. A Food Propensity questionnaire will be administered to document recent trends in food choices, and the exercise and hunger questionnaires will also been done. Following completion of baseline assessments, randomization to amantadine or placebo will occur. Subjects will begin taking 1 capsule daily for 2 days, 2 capsules daily for 5 days, and then 3 capsules daily. Amantadine capsules will each contain 100 mg of amantadine. Clinical state and emergence of side effects, including orthostatic hypotension, will be monitored at all visits by research clinicians. If subjects experience mild side effects with increase in number of capsules, the titration will be slowed. During Phase 2, subjects will be called by research personnel on 3 nonconsecutive days to complete a 24 hour diet recall by phone. Biweekly, all subjects will meet with a study physician or nurse for standardized healthy lifestyle counseling based on the Solutions for Wellness program available from Lilly. BMI will be calculated biweekly and baseline measures will be repeated at the final study visit. Urine pregnancy test will be performed monthly on all women. When subjects gain an additional 3 BMI units (approximately 15 lbs), or when they complete 16 weeks of double blind medication they will repeat all Visit 13 measures and end study participation.