Reduction of human experimental muscle pain by alfentanil and morphine

Musculoskeletal pain is a major clinical problem. By using various experimental models in humans, the understanding of the basic mechanisms behind muscle pain can increase, thereby giving hope for new and optimized treatment. Opioids are increasingly often used to treat muscle pain. There are, however, a limited number of previous studies on opioids and muscle pain, most of them using a relative low, single dose. Therefore, we wanted to further study the effect of two rather high doses of alfentanil (25 and 75ng/ml) and morphine (0.14 and 0.28mg/kg) in human volunteers. The study consisted of two parallel studies with morphine and alfentanil, respectively, and was conducted as randomized, double-blinded, placebo-controlled, 3-way cross-over. We used intramuscular infusion of hypertonic saline and intramuscular electrical stimulation to induce experimental pain. Visual analog scale (VAS)-score, intramuscular electrical pain thresholds and pain area (local and referred) were measured. Both alfentanil and morphine at their highest doses induced a 6 to 7-fold increase in pain thresholds to single and repetitive (5 stimulations, 2Hz) electrical stimulation. Alfentanil and morphine also reduced VAS score about 4 to 5-fold during suprathreshold electric stimulation and during infusion of hypertonic saline. None of the drugs decreased referred pain. There were no apparent differences between the drugs, in terms of effect or adverse reactions. In conclusion, this is the first study to compare two high doses of alfentanil and morphine on experimental muscle pain in humans. Both alfentanil and morphine reduced experimental muscle pain. There were no indications of any true pharmacodynamic differences between the two drugs.