Men and women can’t feel each other’s pain. Literally. We have different biological pathways for chronic pain, which means pain-relieving drugs that work for one sex might fail in the other half of the population.

So why don’t we have pain medicines designed just for men or women? The reason is simple: Because no one has looked for them. Drug development begins with studies on rats and mice, and until three years ago, almost all that research used only male animals. As a result, women in particular may be left with unnecessary pain—but men might be too.

Now a study in the journal Brain reveals differences in the sensory nerves that enter the spinal cords of men and women with neuropathic pain, which is persistent shooting or burning pain. The first such study in humans, it provides the most compelling evidence yet that we need different drugs for men and women.

"There’s a huge amount of suffering that’s happening that we could solve," says Ted Price, professor of neuroscience at the University of Texas, Dallas, and an author of the Brain article. “As a field, it would be awesome to start having some success stories.”

Modern-day pain control is notoriously dismal. Our go-to medicines—opioids and anti-inflammatories—are just new versions of opium and willow bark, substances we’ve used for thousands of years. Although they are remarkably effective in relieving the sudden pain of a broken bone or pulled tooth, they don’t work as well for people with persistent pain that lasts three months or longer.

Some 50 million people struggle with pain most days or every day, and chronic pain is the leading cause of long-term disability in the United States. Women are more likely than men to have a chronic pain condition, such as arthritis, fibromyalgia, or migraines.

Meanwhile, pain medications are killing us. About 17,000 people die each year from prescribed opioids as clinicians write almost 200 million opioid prescriptions, or more than one for every two American adults.

The failure to include sex differences in the search for better pain relief stems in part from flawed but deep-seated beliefs. “[Medical researchers] made the assumption that men and women were absolutely identical in every respect, except their reproductive biology,” says Marianne Legato, a cardiologist who began sounding an alarm in the 1980s about differences in heart attack symptoms among women. She went on to pioneer a new field of gender-specific medicine.

The physiology of pain is just one of many ways that men and women differ, she says. But she isn’t surprised that no sex-specific medicines have emerged. The medical community—including pharmaceutical companies—didn’t appreciate the variation between men and women, including in their metabolisms, immune systems, and gene expression. "If there were differences in how their drugs worked between men and women, they didn’t want to hear about it," she says.

The Brain study came about from a unique opportunity at M.D. Anderson Cancer Center in Houston. You can’t take a biopsy of spinal tissue, but researchers were able to study clusters of sensory neurons in eight women and 18 men who had spinal tumors removed. The analysis included sequencing RNA to determine which genes are active in the neural cells. They compared men and women who had a history of chronic neuropathic pain to those who didn’t. Their pain wasn’t caused by the tumors themselves. Some patients had nerve compression causing neuropathic pain, while others didn’t have neuropathic pain or chronic pain at all.