[An atypical opioid analgesic: tramadol].Agri. 2006 Jan; 18(1):5-19.AGRI
Tramadol, a centrally acting analgesic, consists of two enantiomers, both of which contribute to analgesic activity via different mechanisms. (+) Tramadol and the metabolite (+) -O- desmethyl-tramadol (M1) are agonists of the mu opioid receptor. (+) Tramadol also stimulates presinaptic release of serotonin and inhibits serotonin reuptake whereas (-) tramadol inhibits norepinephrine reuptake. Thus tramadol enhances inhibitory effects on pain transmission both by opioid and monoaminergic mechanisms. The complementary and synergistic actions of the two enantiomers improve the analgesic efficacy and tolerability profile of the racemate. Following oral administration the bioavailability of tramadol is high and with new slow release preparations twice daily administration enables effective pain control. The recommended maximum daily dose of tramadol is 400 mg/day. Tramadol is characterised by low plasma protein binding and quite extensive tissue distribution. Elimination is primarily by the hepatic route (metabolism by CYP2D6) and partly by the renal route. It is effective in different types of moderate-to-severe acute and chronic pain, including neuropathic pain, low back pain, osteoarthritis pain and breakthrough pain. It also causes fewer opioid-type adverse effects, e.g. nausea, drowsiness, vomiting, dry mouth and constipation. Although trials in literature demonstrate immune-stimulating effects of tramadol, there are also trials suggesting immunesuppressive effects that are lesser than morphine. Owing to its pharmacological properties, tramadol is more appropriate than NSAIDs for patients suffering from gastrointestinal and renal problems. Besides its proven clinical efficacy tramadol is a safe drug as respiratory depression, cardiovascular side effects, drug abuse and dependence are of minor clinical relevance, unlike some other opioids.