Not only do men and women feel pain differently, but medication that relieves men's pain won't necessarily work as well -- or at all, in some cases -- in women. Until 1993, the U.S. Food and Drug Administration (FDA) regulations excluded "women of childbearing age" from participating in clinical trial phases for new drugs, which meant that pharmaceutical companies often only tested on men [source: Toomey]. As a result, many painkiller formulations and dosages were developed based on the male body's pain response.
Morphine, one of the most commonly used painkillers, doesn't perform as effectively in both genders. For surgery patients undergoing general anesthesia, females require at least 30 percent more morphine than males [source: Toomey]. In addition, nalbuphine, which is often used to anesthetize women during childbirth, does little to mitigate pain in men [source: Economist].
Scientists attribute those gendered effects to variations in male and female brains. Both medications relieve pain by activating opioid receptors in the brain; opioid receptors counteract pain by blocking neurons from transmitting pain signals. Yet, morphine binds to a class of opioid receptors called mu receptors, and nalbuphine binds to kappa receptors. PET scans have shown, however, that mu receptors in healthy male and female brains activate through different pathways [source: Toomey]. Moreover, the male mu opioid system plays the starring role in pain relief, while the kappa system dominates in females.
Since drug testing was primarily focused on male pain responses in the past, the most popular painkillers were designed to stimulate mu opioids. As a result, chronic and severe pain in women has been harder to treat successfully. But now that scientists recognize that kappa receptor opioids better stifle pain in women, some are designing so-called "pink and blue painkillers" specifically tailored to males' and females' unique pain response systems [source: Economist]. If that becomes a medical reality, perhaps the gender pain gap will narrow in the future.