For years, researchers assumed that women have a higher pain threshold and are less sensitive to pain than men. After all, men don't have to pass a head the circumference of a large grapefruit through an opening that starts at the diameter of a Cheerio as women do to give birth.
But recent research has called into question the assumption that women have greater pain tolerance. Science is providing new insights into pain relief, anesthesia and, oddly enough, redheads.
Jeffrey Mogil, Ph.D., is professor of pain studies at McGill University in Montreal, Canada. He explains that while not all studies have found sex differences, those which have all point in the same direction that contradicts conventional wisdom. "Females are more sensitive to pain, less tolerant and more able to discriminate different levels of pain than males," he says. This is true in studies of both humans and animals.
Women are also much more likely to suffer from chronic pain conditions than men. Researchers originally suspected that this was primarily due to the fact that they are more likely to seek medical care in general. But while women do indeed seek more care, they're also genuinely more likely to develop painful conditions like fibromyalgia, rheumatoid arthritis and migraines. For example, 80 to 90 percent of people with fibromyalgia are women, according to the National Institute of Arthritis and Musculoskeletal and Skin Diseases.
Pregnancy actually is the exception to the rule. While pregnant, women do become progressively less sensitive to pain as they get closer to giving birth. Natural painkillers like endorphins are elevated during pregnancy and labor, helping fight pain.
Says Mogil, studies show "there is [pregnancy-related pain relief] in both rats and humans," but, he adds, very little research has been done on this in recent decades. He also notes that it's not very effective in humans, given the amount of pain that most women still suffer.
Scientists have discovered that women's experience of pain may be not only quantitatively different, but qualitatively as well. And here's where the redheads come in.
Anesthesiologists had long reported that redheads tended to need more anesthesia than others. But until 2002, no studies had been presented on the subject. Up until that point, it could have just been a clinical myth prompted by a few memorable anesthesia-resistant Gingers. However, when a research group in Louisville, Ky., studied the phenomenon, they found that redheads did indeed need larger doses of a common anesthetic—about 20 percent higher, on average—to blunt the pain from electrical shock.
This finding turned out to only be part of the story.
In research conducted by Mogil's group on opioid medications, only redheaded women were different from others in the way these drugs relieved their pain. Plus they needed less opioid painkillers than brunettes or blondes. Male redheads were the same as everyone else.
Why would this be?
It turns out that processing pain involves a receptor which, when mutated, produces red hair and fair skin, among other changes. This receptor—the MC1r receptor— was initially thought to occur only in the skin. However, it has now been found in the brain, and there, in women, it's part of the pathway that processes pain.
"In males, an NMDA receptor processes pain that can be modulated by opioids," says Mogil. "In females, there appear to be no NMDA receptors involved. Instead, women use the MC1r receptors." In redheads, that receptor is mutated, and this makes opioids work more efficiently for them.
In fact, Mogil says that opioids probably work better for women than men in general—and this may be due to the different pathways involved.
Overall, the research so far shows how complicated pain is—the genetics that underlie the effects of anesthesia (which kills pain by causing numbness or unconsciousness) are different from those that determine the effects of analgesia (which reduces or eliminates pain without numbness or unconsciousness).
These findings also mean that both genders need to be studied in pain research so that any differences can be discovered—and accounted for in treatment.
What does this mean if you're a redhead?
Natural redheads undergoing surgery should be sure that their physicians know that their hair is naturally that color—and people who dye their hair red may want to be sure their anesthesiologists are aware of this as well, so that anesthesia doses can be adjusted accordingly if needed.
When it comes to using opioids, carrot tops should be careful to start with the lowest effective dose. It's possible that increased sensitivity could mean that they are at higher risk for overdose, though this has not been proven.