Tramadol is a weak μ-receptor agonist only partially antagonized by naloxone. Its analgesic activity is mainly based on blockade of the reuptake of serotonin; it is a weak norepinephrine reuptake blocker. Tramadol is effective in treatment of moderate pain and has been used as an adjunct to opioid analgesics in chronic pain syndromes. The drug is relatively contraindicated in patients with a history of seizure disorders, and there is risk of the serotonin syndrome if it is co-administered with SSRIs. No significant effects on cardiovascular functions or respiration have been reported.