Pharmacotherapeutic group: Opioid analgesics, other opioids
ATCvet Code: QN02AX02
Pharmacodynamic properties
Tramadol is a centrally acting analgesic agent with a complex mode of action exerted by its 2 enantiomers and primary metabolite, involving opioid, norepinephrine, and serotonin receptors. The (+) enantiomer of tramadol has a low affinity for the µ-opioid receptors, inhibits serotonin uptake and enhances its release. The (‑) enantiomer preferentially inhibits norepinephrine reuptake. The metabolite O-desmethyltramadol (M1) has greater affinity for the µ-opioid receptors.
Unlike morphine, tramadol does not have depressing effects on respiration for an extensive analgesic dose range. Likewise, it does not affect gastrointestinal motility. The effects on the cardiovascular system tend to be mild. The analgesic potency of tramadol is about 1/10-1/6 of that of morphine.
Pharmacokinetic properties
Tramadol is readily absorbed: After a single oral administration of 4.4 mg tramadol HCL per kg body weight, peak plasma concentrations of 65 ng tramadol per mL are achieved within 45 minutes. Food does not significantly affect the absorption of the drug.
Tramadol is metabolized in the liver by cytochrome P450 mediated demethylation followed by conjugation with glucuronic acid. In dogs, lower levels of the active metabolite O-desmethyltramadol are formed compared to humans. Elimination occurs mainly via the kidneys with an elimination half-life of about 0.5-2 hours.