Women May Be More Adept Than Men At Discerning Pain

Enlarge this image Grace Heejung Kim for NPR Grace Heejung Kim for NPR

The pathway to opioid abuse for women often starts with a prescription from the doctor's office. One reason is that women are more likely than men to seek help for pain.

Pain researchers say that not only do women suffer more painful conditions, they actually perceive pain more intensely than men do.

"The burden of pain is substantially greater for women than men," says researcher and psychologist Roger Fillingim, "and that led pain researchers like myself to wonder if the pain perception system is different in women than in men."

For more than two decades, Fillingim has been studying gender differences and pain, most recently at the University of Florida's Pain Research and Intervention Center of Excellence, where he is director. He recruits healthy male and female volunteers to take part in experimental pain sessions using various painful stimuli, including pressure, heat, cold and electrical stimulation.

Probes are typically applied to the hand or arm. As intensity of the stimuli is increased, volunteers are asked to rate their pain on a scale of zero to 10, where zero is no pain and 10 is the most intense pain one can imagine. If volunteers report pain levels at 10, Fillingim stops the experiment immediately.

Enlarge this image toggle caption Raymon Troncoso for NPR Raymon Troncoso for NPR

In comparing the responses between women and men, he says the findings in his studies and those in other pain research are consistent.

"On average, women report the same stimuli to be more painful than men," Fillingim says, emphasizing that the same stimulus is applied to everybody, so if there are differences in how painful the experience is, it can't be because of the stimulus because it's calibrated to be the same for all.

Now it could be that men are less willing to admit they have pain. It could also be that women have learned to cope with pain and are more accustomed to talking about it. After all, don't women experience more pain in their lives? What about the monthly pain of menstruation? Or the extreme pain of childbirth?

According to Fillingim, many women — if not most — experience through childbirth what appears to be "one of the most painful experiences in normal life," he says, "but that doesn't speak to pain sensitivity" in the lab, for example. So "experiencing an exquisitely painful event is just a different question than whether or not men and women differ in their pain responses."

For pain researchers such as Fillingim, pinpointing how and why men and women respond differently to pain is an important first step toward finding more effective treatments for pain. It might be that medicines should be tailored according to sex or gender.

"We may ultimately need pink and blue pills, but in order to get there we need to understand what the mechanisms are that are female-specific or male-specific so that we can design more personalized therapies that are going to help reduce pain for women and men in the long run."

That would be a welcome development according to psychologist Carolyn Mazure of the Yale School of Medicine.

"Women are more likely, for example, to have chronic headache, lower back pain and neck pain; women are more likely to have a chronic disability of one kind or another which often is associated with pain," says Mazure. "So whether or not women report pain more or have pain more, I think we could say that both may be true."

And when women go to the doctor they're more likely to be prescribed opioids than men. Mazure says this can be especially dangerous because the progression from exposure to an addictive substance to addiction is more rapid for women than it is for men. "We know that with regard to opioids, cocaine, alcohol and even smoking tobacco," Mazure says.

This is complicated by the fact that women suffer more anxiety and depression than men do. That means women with these conditions are often prescribed other drugs that can result in dangerous combinations that increase the risk of overdose.

In fact, federal data show that while more men die from drug overdoses than women do, the rate of death among women is accelerating faster than it is among men.

Mazure says understanding the "why" behind differences in pain perception could help researchers come up with better treatments.

toggle caption Raymon Troncoso for NPR

As it is now, pain researchers such as Fillingim can only speculate about what might trigger differences in pain perception between women and men. There are numerous possibilities. One, he says, is biological and might have to do with sex hormones.

"Women have both higher levels and fluctuations in circulating estrogens and progesterone, and those may contribute to experiencing higher levels of pain," Fillingim says, "whereas men have higher levels of testosterone," which in some studies has been shown to be protective against pain or associated with lower pain sensitivity.

Another possible contributor could have to do with increased susceptibility to anxiety, depression and sadness — all of which are known to increase sensitivity to pain.

It could also have to do with evolution. For example, in hunter-gatherer societies, men were the hunters and women took care of the village and raised the children so that they could successfully pass on their genes. So the pressures for men and women were different.

"Women were more vigilant to a variety of environmental stimuli that might represent danger to them, and particularly to their children," Fillingim says. "Those women who were able to respond and detect those threats and deal with them successfully survived, and their genes are more represented in the gene pool."

Men, on the other hand, might have been less bothered by painful experiences so they could persevere and kill whatever they needed to kill to bring it home so everybody could eat, he says. Those evolutionary pressures could have some representation in today's modern society when it comes to the perception of pain.

One question researchers are only starting to look into is whether transitioning from one sex to another makes a difference in how people perceive pain.

Fillingim points to the findings in one study that suggest individuals transitioning from male to female experienced more pain after the transition, whereas for individuals transitioning from female to male, there weren't many changes in pain after the transition.

"Whether that was a result of the hormonal therapies that were part of the transition or the surgical experience or other factors, I don't think we really know," he says.

It's a fascinating new area of study, Fillingim says, and one that will likely become increasingly important in coming years.