Naloxone changes self-ratings but not performance in normal subjects.

The effects of single intravenous doses of naloxone (0.8 and 1.6 mg) in a variety of performance tasks and on subjective ratings of mood and bodily symptoms were investigated in 12 student volunteers. Naloxone was without effect on any of the performance measures. However, 5 min after naloxone (1.6 mg) the subjects felt significantly more troubled, mentally slow, incompetent, withdrawn and physically tired, and less irritable. These effects appeared to be dose-related since 0.8 mg produced similar, but not statistically significant changes. Sixty-five minutes after the higher dose subjects felt significantly more muzzy and incompetent; in contrast to the effects at 5 min they now felt significantly more irritable. These results are difficult to explain solely in terms of opiate receptor blockade.