The hydrochloride salt of the semi-synthetic opioid hydromorphone with analgesic activity. Hydromorphone, the hydrogenated ketone of morphine, selectively binds the mu-opioid receptor, a G protein-coupled receptor. Binding stimulates the exchange of guanosine triphosphate (GTP) for guanosine diphosphate (GDP) on the G-protein complex, resulting in inhibition of plasma membrane-associated adenylate cyclase (AC) and a reduction in intracellular cyclic 3',5'-adenosine monophosphate (cAMP) levels. Due to a reduction in cAMP levels, voltage-gated potassium channels are activated, resulting in neuronal hyperpolarization and a reduction in neuronal excitability. In addition, this agent inhibits the opening of voltage-gated calcium channels, resulting in inhibition of calcium entry into neuronal cells and a reduction in the release of nociceptive neurotransmitters such as substance P and glutamate.