Samidorphan HCl is an orally active and highly potent modulator of the opioid system that binds to μ‐opioid, κ‐opioid, and delta-opioid receptors. Samidorphan HCl is a novel μ-opioid receptor antagonist, a partial agonist for k-opioid and delta-opioid receptors. Samidorphan HCl can be used to prevent and treat schizophrenia.