Levine was just testing painkillers on people who'd had a wisdom tooth extract- ed, when he uncovered rather more than he'd bargained for.The women in his study group found that strong painkillers related to morphine, called kappa-opioids, were most effective at numbing pain.'In fact, the doses used in the clinical trial made pain worse for men,' says Levine, a clinical neuroscientist from the University of California in San Francisco.But the same drugs didn't work for the men at all.