Tramadol is a centrally acting opioid analgesic used to treat moderate to moderately severe pain. The drug has a wide range of applications, including treatment of rheumatoid arthritis, restless legs syndrome, motor neurone disease and fibromyalgia. Tramadol is a serotonin releaser, reuptake inhibitor of norepinephrine and a weak μ-opioid receptor agonist. Tramadol is metabolized to O-desmethyltramadol, a significantly more potent μ-opioid agonist. Tramadol and its major metabolite(s) are distinguished from other more potent opioid agonists by relative selectivity for μ-opioid receptors. Tramadol is used similarly to codeine, to treat moderate to severe nerve pain. Pharmacologically, tramadol is similar to levorphanol (albeit with much lower μ-agonism), both agents have SNRI activity. Tramadol is also molecularly similar to venlafaxine (Effexor) and has similar SNRI effects, with antinociceptive effects.