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The opioid agonist and antagonist properties of tramadol were assessed in 6 male opioid-dependent volunteers enrolled in a methadone maintenance programme. Subjects participated in 3 experimental sessions in which the effects of intramuscular tramadol 100 and 300mg and placebo were evaluated. Tramadol neither produced morphine-like effects nor precipitated a withdrawal syndrome; its subjective, behavioural and physiological effects were not different from those of placebo. Although the results of this study suggest that tramadol has a low abuse liability in opioid-dependent subjects, higher doses should be tested to confirm these data.