In summary, the present study suggests that doses of mitragynine ranging between 5 and 40 mg were well tolerated in a controlled setting and that low doses (5–10 mg) of mitragynine may cause subjective feelings of stimulation and increased attention, pointing in the direction of enhancement of performance. On the other side, the highest dose (40 mg) might also produce inhibitory effects resulting in amnesia and psychopathological distress.