Tramadol (sold under the brand name Ultram) is a narcotic analgesic proposed for moderate to severe pain. Tramadol and its O-desmethyl metabolite (M1) are selective, weak OP3-receptor agonists. Opiate receptors are coupled with G-protein receptors and function as both positive and negative regulators of synaptic transmission via G-proteins that activate effector proteins. As the effector system is adenylate cyclase and cAMP located at the inner surface of the plasma membrane, opioids decrease intracellular cAMP by inhibiting adenylate cyclase. Subsequently, the release of nociceptive neurotransmitters such as substance P, GABA, dopamine, acetylcholine, and noradrenaline is inhibited. The analgesic properties of Tramadol can be attributed to norepinephrine and serotonin reuptake blockade in the CNS, which inhibits pain transmission in the spinal cord. The (+) enantiomer has the higher affinity for the OP3 receptor and preferentially inhibits serotonin uptake and enhances serotonin release. The (-) enantiomer preferentially inhibits norepinephrine reuptake by stimulating alpha(2)-adrenergic receptors. Tramadol is used primarily to treat mild-severe pain, both acute and chronic. Its analgesic effects take about one hour to come into effect and 2 h to 4 h to peak after oral administration with an immediate-release formulation. On a dose-by-dose basis, tramadol has about one-tenth the potency of morphine and is approximately equally potent when compared to pethidine and codeine. The most common adverse effects of tramadol include nausea, dizziness, dry mouth, indigestion, abdominal pain, vertigo, vomiting, constipation, drowsiness, and headache. Compared to other opioids, respiratory depression and constipation are considered less of a problem with tramadol.

O-Desmethyl tramadol (O-Desmethyltramadol, O-DSMT) is a metabolite of tramadol. O-Desmethyltramadol is an opioid analgesic and the main active metabolite of tramadol. (+)-O-Desmethyltramadol is the most important metabolite of tramadol produced in the liver after tramadol is consumed. This metabolite is considerably more potent as a μ-opioid agonist than the parent compound. O-desmethyl tramadol, inhibits 5-hydroxytryptamine type 2C receptors expressed in xenopus oocytes. O-desmethyl tramadol inhibits functions of M(1) receptors but has little effect on those of M(3) receptors. O-desmethyl tramadol has been widely used clinically and has analgesic activity.