Effexor (venlafaxine) is used to treat major depressive disorder in adults. Effexor XR, the extended-release version, is also used for generalized anxiety disorder, social anxiety disorder, and panic disorder. It belongs to a class of medications called SNRIs (selective serotonin and norepinephrine reuptake inhibitors). Venlafaxine works by increasing the levels of serotonin and norepinephrine in the brain, which help maintain mental balance.