Fatal overdoses of tramadol: is benzodiazepine a risk factor of lethality?

Tramadol is a centrally acting analgesic agent used in the treatment of mild to moderate pain. It has a low affinity to opioid receptors and inhibits the reuptake of norepinephrin and serotonin producing an analgesic action by blocking nociceptive impulses in the spine. Although 21 drug-combined fatalities including tramadol have been reported, only two fatal overdoses in adults with tramadol alone have been reported to date. We report four additional lethal intoxications, assess the toxicity of tramadol, the detection method and the possible interaction with other central nervous system (CNS) depressants, particularly benzodiazepines. Similarities between tramadol and buprenorphine are discussed, and a possible cytochrome P450-based interaction between tramadol and benzodiazepine is considered. To our knowledge, this relationship has never been reported in the literature.