Paroxetine hydrochloride and paroxetine mesylate belong to a class of antidepressant agents known as selective serotonin-reuptake inhibitors (SSRIs). Despite distinct structural differences between compounds in this class, SSRIs possess similar pharmacological activity. As with other antidepressant agents, several weeks of therapy may be required before a clinical effect is seen. SSRIs are potent inhibitors of neuronal serotonin reuptake. They have little to no effect on norepinephrine or dopamine reuptake and do not antagonize α- or β-adrenergic, dopamine D2 or histamine H1 receptors. During acute use, SSRIs block serotonin reuptake and increase serotonin stimulation of somatodendritic 5-HT1A and terminal autoreceptors. Chronic use leads to desensitization of somatodendritic 5-HT1A and terminal autoreceptors. The overall clinical effect of increased mood and decreased anxiety is thought to be due to adaptive changes in neuronal function that leads to enhanced serotonergic neurotransmission. Side effects include dry mouth, nausea, dizziness, drowsiness, sexual dysfunction and headache (see Toxicity section below for a complete listing of side effects). Side effects generally occur during the first two weeks of therapy and are usually less severe and frequent than those observed with tricyclic antidepressants. Paroxetine hydrochloride and mesylate are considered therapeutic alternatives rather than generic equivalents by the US Food and Drug Administration (FDA); both agents contain the same active moiety (i.e. paroxetine), but are formulated as different salt forms. Clinical studies establishing the efficacy of paroxetine in various conditions were performed using paroxetine hydrochloride. Since both agents contain the same active moiety, the clinical efficacy of both agents is thought to be similar. Paroxetine may be used to treat major depressive disorder (MDD), panic disorder with or without agoraphobia, obsessive-compulsive disorder (OCD), social anxiety disorder (social phobia), generalized anxiety disorder (GAD), post-traumatic stress disorder (PTSD) and premenstrual dysphoric disorder (PMDD). Paroxetine has the most evidence supporting its use for anxiety-related disorders of the SSRIs. It has the greatest anticholinergic activity of the agents in this class and compared to other SSRIs, paroxetine may cause greater weight gain, sexual dysfunction, sedation and constipation.