Tramadol (Ultram) is an analgesic agent with multiple mechanisms of action. It is a weak /-agonist with approximately 30% of the analgesic effect antagonized by the opioid antagonist naloxone. Used at recommended doses, it has minimal effects on respiratory rate, heart rate, blood pressure, or GI transit times. Structurally, tramadol resembles codeine with the B, D, and E ring removed. The manufacturer states that patients allergic to codeine should not receive tramadol, because they may be at increased risk for anaphylactic reac-tions.99 Tramadol is synthesized and marketed as the racemic mixture of two (the [2S, 3S] [-] and the [2R, 3R] [ + ]) of the four possible enantiomers.100 The (+) enantiomer is about 30 times more potent than the (—) enantiomer; however, racemic tramadol shows improved tolerability.101,102 Neurotransmitter reuptake inhibition is also responsible for some of the analgesic activity with the (—) enantiomer primarily responsible for norepinephrine reuptake and the (+ ) enantiomer responsible for inhibiting serotonin reup-take.101,103 Like codeine, tramadol is O-demethylated via CYP2D6 to a more potent opioid agonist having 200-fold higher affinity for the opioid receptor than the parent compound. Tramadol was initially marketed as nonaddictive, and a 3-year follow up study showed that the abuse potential is very low, but not zero. Most abusers of tramadol have abused opioid drugs in the past.104 Both enantiomers of tramadol and the major O-demethylated metabolite are proconvulsive, and tramadol should not be used in patients with a low-seizure threshold including patients with epilepsy.101

Mixed Agonist/Antagonist


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