In an experiment revealing significant and unexplained differences between the sexes, researchers at the University of California at San Francisco have discovered that a class of drugs long neglected in the treatment of severe pain is far more effective and long-lasting in women than in men.

The unexpected results are already sending the scientists back to the laboratory in an effort to decipher the differences in the way men and women perceive pain and why the receptor cells in their brains appear tuned so differently to the different classes of pain relievers called opioid drugs.The researchers agree that the results are surprising largely because virtually all past experiments on the effects of pain and pain relief have been performed on men. That fact, the scientists say, should demand intensified research aimed at understanding more of the fundamental differences - hormonal and physiological, as well as anatomical - between the sexes.

The new report, published Wednesday in the international journal Nature Medicine, comes from a research team headed by Dr. Jon D. Levine, professor of medicine and oral surgery at UCSF, who for more than 20 years has been studying the complex pathways that carry signals from painful injuries and diseases through the central nervous system to the brain as well as its natural pain-relievers, known as endorphins.

The researchers studied 48 young men and women who had undergone painful surgery to remove impacted wisdom teeth and who were treated with drugs known as kappa opioids - a class that includes well-known prescription compounds such as Nubain, Talwin and Stadol. Past research had convinced physicians that kappa opioids are far less effective than the more-powerful class, known as mu opioids, which include morphine, Demerol, codeine and the illegal narcotic heroin.

In fact, Levine and his colleagues found, the women experienced major pain relief from the kappa opioids, while the men did not.

"The results were most dramatic," Levine said in an interview Tuesday. "While the men had a very weak response to the drugs, the women not only experienced great pain relief, but the relief lasted surprisingly long."