Scientists who study chronic pain have long puzzled over a particular type of scenario: A man and woman get into a car accident and sustain the same injuries. But the woman experiences long-lasting pain, while the man recovers more quickly.
Historically, some doctors have dismissed these differences as women exaggerating their pain or being unable to tolerate the same discomfort as men. But studies have repeatedly found that women are more likely to experience chronic pain in general and that their pain lasts longer, on average.
A study published Friday in the journal Science Immunology offers a clue as to why: Men’s immune systems may have a better mechanism for shutting off pain, likely because of their higher testosterone levels.
“What we show is, it’s a real biological mechanism from the immune cells. It’s not in the mind,” said Geoffroy Laumet, one of the study’s authors and an associate professor of physiology at Michigan State University.
Ann Gregus, an assistant professor at Virginia Tech who researches ways to treat chronic pain, said the findings underscore the need to take women’s pain seriously.
“A lot of women are taught to hide their pain, because then, if they don’t, people will perceive that they can’t do their jobs, that they can’t take care of their families,” Gregus, who wasn’t part of the new study, said.
The findings apply to persistent pain after surgery or a physical trauma. However, other chronic pain conditions — like fibromyalgia — don’t follow a traumatic event.
“Does it explain everything? I don’t think so. We don’t have any single, magical pathway,” said Dr. Michele Curatolo, a professor of anesthesiology and pain medicine at the University of Washington, who wasn’t involved in the new research. Curatolo is the chief medical officer at 4E Therapeutics, a neuroscience company developing treatments for pain.
For the new study, the researchers asked 245 people who had experienced traumatic injuries — primarily car accidents — to rate their level of pain. Though men and women had roughly the same pain severity on the day the injury occurred, men saw their pain resolve more quickly over a period of nearly three months.
Blood tests showed that the men had higher levels of a molecule called interleukin-10, which switches off pain signals to the brain. Laumet said testosterone increased the production of interleukin-10 from white blood cells.
The same was true in lab experiments involving mice.
The team of scientists gave mice injections to stimulate an inflammatory response, and afterward, male mice showed signs of their pain resolving, whereas female mice did not. Male mice also recovered faster from a small surgical incision and from being restrained in a tube for two hours — a scenario designed to mimic the physical and emotional stress of a car accident.
Across the experiments in mice, white blood cells that produce interleukin-10 were far more active in males.
Gregus said the new research likely reflects evolutionary differences between men and women.
“It’s building upon a series of studies that have been done to show that men tend to use their innate immune system more effectively than women,” she said. (The innate immune system is the body’s first line of defense against foreign invaders.)
But Curatolo added that the sex differences aren’t black and white — and men often have long-lasting pain, too.
In the long run, Laumet said, the findings could be used to develop new treatments for chronic pain in women, such as testosterone patches. Topical treatments like that tend to come with fewer side effects than systemic drugs, he said.
As it stands, many of the current options for chronic pain relief have concerning side effects and don’t fully alleviate symptoms. Long-term use of over-the-counter painkillers can lead to kidney damage or stomach ulcers. Opioids, often prescribed as a last resort, can be addictive. Doctors even sometimes prescribe antidepressants or anti-seizure medications off-label for pain, but many patients with chronic pain don’t respond to them. And physical therapy or acupuncture may work for some patients but not others.
“We clearly need alternative options,” Gregus said.
