Double-blind administration of a single intramuscular dose of 1.5, 2.0, 2.5, or 3.0 mg levonantradol or placebo to 56 patients with moderate to severe postoperative or trauma pain showed significant analgesic effects of each dose of levonantradol as compared to placebo (P less than 0.05). However, no significant dose response was observed. Compared to 2/16 patients on placebo, 23/40 patients (57 per cent) on levonantradol reported one or more side effect. Drowsiness was most frequent. Dry mouth, dizziness, "weird dreams," mild hallucinations, nervousness, apprehension and confusion occurred less frequently. Changes in resting heart rate and blood pressure were minor and general acceptability was good.