Study to Evaluate the Efficacy of ALKS 3831 on Body Weight in Young Adults Who Have Been Recently Diagnosed With Schizophrenia, Schizophreniform, or Bipolar I Disorder

Inclusion Criteria: * Has less than 24 weeks previous treatment with antipsychotics (cumulative; lifetime) * Subject treated with aripiprazole can receive an additional 1 year of treatment at ≤5 mg/day, and this treatment will not be considered as part of the 24 weeks of previous treatment with antipsychotics * Has less than 4 years elapse since the initial onset of active-phase of symptoms * Has a body mass index (BMI) of \<30 kg/m\^2 * Agrees to use an acceptable method of contraception for the duration of the study and for 30 days after the last dose of study drug * Subject meets the criteria for a primary diagnosis of schizophrenia, schizophreniform disorder, or bipolar I disorder * For bipolar I disorder, must have been experiencing an episode of acute mania within ≤14 days prior to Visit 1 * Suitable for outpatient treatment * Additional criteria may apply Exclusion Criteria: * Poses a current suicide risk * Has a history of poor or inadequate response to treatment with olanzapine * Has previously been treated with long-acting injectable antipsychotic medication within the 2 months prior to screening, or has \> 6 months cumulative life use, or has received treatment with electroconvulsive therapy in their lifetime * Has initiated treatment with mood stabilizers (eg lithium, valproate, etc) \>2 months prior to Visit 1 * Has a positive drug screen for opioids, phencyclidine (PCP), amphetamine/methamphetamine, or cocaine * Has taken opioid agonists (eg, codeine, oxycodone, tramadol, morphine) within the 14 days prior to Visit 1, or has taken opioid antagonists, including naltrexone and naloxone, within 60 days prior to Visit 1 * Taking any weight loss agents or hypoglycemic agents * Has a clinically significant or unstable medical illness, condition, or disorder that would be anticipated to potentially compromise subject safety or adversely affect the evaluation of efficacy * Has joined a weight management program or had significant changes in diet or exercise regimen within the past 6 weeks * Has started a smoking cessation program within the past 6 months * Has a history of diabetes * Currently pregnant or breastfeeding or is planning to become pregnant during the study or within 30 days of the last study drug administration * Additional criteria may apply