Tramadol hydrochloride is a centrally-acting synthetic 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 and fibromyalgia. Tramadol is a very weak μ-opioid receptor agonist, induces serotonin release, and inhibits the reuptake of norepinephrine. Tramadol is converted to O-desmethyltramadol, a significantly more potent μ-opioid agonist. The opioid agonistic effect of tramadol and its major metabolite(s) is almost exclusively mediated by such μ-opioid receptors. This further distinguishes tramadol from opioids in general (including morphine), which do not possess tramadol’s degree of receptor subtype selectivity and which are much stronger opiate-receptor agonists.