Venlafaxine is a selective serotonin-norepinephrine reuptake inhibitor (SNRI) used in the palliative care setting for concurrent treatment of depressive disorders and neuropathic pain. (Neuropathic chronic pain is often resistance to standard opioid therapy and is best treated with a secondary amine tricyclic antidepressant (nortriptyline, desipramine), a selective serotonin or norepinephrine reuptake inhibitor (SSNRI) (venlafaxine, duloxetine), or a calcium channel alpha2 delta lignd (anticonvulsants) (gabapentin or pregabalin).
In the United States there are three SNRIs that have been approved by the FDA: venlafaxine (Effexor and Effexor XR), desvenlafaxine (Pristiq), and duloxetine (Symbalta). Venlafaxine and duloxetine both block the serotonin and norepinephrine transporters, thereby inhibiting and availability to bind with the postsynaptic receptors. At lower doses, venlafazine predominantly affects serotonin reuptak, contributing to greater anxiety reduction more so than depressive symptom reduction. Duloxetine, however, appears to be a more potent and equal serotonin and norepinephrine reuptake inhibitor than venlafaxine is. These drugs are rapidly absorbed after oral intake and metabolized extensively in the liver. Time needed to reach maximum plasma concentration is 2 hours for both venlafaxine and duloxetine. Venlafaxine has a half-life of 5 hours and the active metabolite is 11 hours. Steady state is achieve in 3 to 4 days. Duloxetine has a half-life of 12 hours, reaching steady state in 3 days. Both drugs are excreted mostly in urine.
The most common side effects with both venlafaxine and duloxetine include headache, somnolence, dizziness, insomnia, nervousness, nausea, dry mouth, constipation, and abnormal ejaculation. Appetite and weight decreases may occur. At higher doses, both drugs may contribute to elevated blood pressure. No specific serum level monitoring is available for either of these drugs. With duloxetine, liver function should be monitoered once weekly, once monthly, biannually, and finally annually. All patients taking antidepressants need to be carefully monitored for suicidal risk as well as activation of hypomanic or manic symptoms.