Rationale: Tapentadol is a novel analgesic that activates mu-opioid receptors and blocks norepinephrine reuptake. There is very little information available regarding the non-analgesic pharmacodynamic effects of tapentadol. Objectives: This outpatient study evaluated the physiological, subject-rated, and performance effects of therapeutic doses of tapentadol compared to two control drugs in humans. Methods: This double-blind, within-subject study examined the effects of oral placebo, tapentadol (25, 50, and 75 mg), tramadol (50, 100, and 150 mg), and hydromorphone (2, 4, and 6 mg). Nine occasional opioid users completed the study. Pharmacodynamic drug effects were measured before and for 6 h after drug administration. Results: All three doses of the tested drugs produced comparable, time-dependent decreases in pupil diameter, but the effects were generally not dose dependent. The high dose of tapentadol, as well as all three doses of tramadol and hydromorphone, increased positive subject-rated effects (e.g., "Good Effects" and "Like the Drug") as a function of time. Only tramadol increased negative subject-rated effects (e.g., "Bad Effects" and "Nauseous"); however, these were of low magnitude. Conclusions: The highest tested dose of tapentadol produced a profile of positive effects comparable to that of hydromorphone, whereas tramadol produced positive and negative subject-rated effects. The mixed findings for tramadol are consistent with previous findings indicating that it has a distinct profile of effects relative to prototypic opioids. Future research should examine the effects of higher tapentadol doses, as well as the factors contributing to the different subject-rated profile of effects observed for tramadol relative to tapentadol and hydromorphone.
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