The hydrochloride salt form of paroxetine, a phenylpiperidine derivative and a selective serotonin reuptake inhibitor (SSRI) with antidepressant and anxiolytic properties. Paroxetine binds to the pre-synaptic serotonin transporter complex resulting in negative allosteric modulation of the complex thereby blocking reuptake of serotonin by the pre-synaptic transporter. Inhibition of serotonin recycling enhances serotonergic function through serotonin accumulation in the synaptic cleft, resulting in long-term desensitization and downregulation of 5HT1 (serotonin) receptors and leading to symptomatic relief of depressive illness. or A rare disorder in which red blood cells are easily destroyed by certain immune system proteins. Symptoms include blood clots, and red or brownish urine in the morning. Aplastic anemia (decreased production of blood cells) may lead to PNH, and people with PNH are at increased risk of acute myelogenous leukemia. Also called PNH.