Samidorphan (ALKS-33) is an orally active opioid system modulator that has a high affinity for binding with μ‐opioid, κ‐opioid, and δ‐opioid receptors. Samidorphan acts as an antagonist at μ‐opioid receptors and acts as a partial agonist at k-opioid and δ‐opioid receptors. Samidorphan primarily acts as an opioid receptor antagonist in vivo. Samidorphan can improve the behavior of depressed animals.