So, we took care of the weight factor. You're right, you do have to account for that. Kappa receptors are not involved in pain, they're involved in the way that opioids work to relieve pain. However, certain painful problems are more common in men than women. For example, women have a higher prevalence of arthritis, migraine and TMJ, where men have a higher incidence of chronic headache, not migraine.
Back pain is probably equal in both genders.
There is some speculation that the pain problems that are more common in women may be hormonally related. Those were the kappa drugs that worked better for women than for men. As people age, the number of pain problems increase. So, as our population ages, we'll see an increase in the prevalence of chronic pain problems, for instance arthritis, degenerative joint disease. Men should not ask for the three drugs I mentioned, pentazocine, nalbuphine and butorphanol.