The hydrochloride salt of a synthetic methyl-piperidine derivative with antihistaminic and anti-serotoninergic properties. Cyproheptadine competes with free histamine (HA) for binding at HA-receptor sites, thereby competitively antagonizing histamine stimulation of HA-receptors in the gastrointestinal tract, large blood vessels, and bronchial smooth muscle. This agent also competes with free serotonin for binding at serotonin receptor sites. Cyproheptadine exhibits anticholinergic and sedative properties and has been shown to stimulate appetite and weight gain.