One of the chief arguments for the legalization of medicinal marijuana is its usefulness as a pain reliever. For many cancer and AIDS patients across the 19 states where medicinal use of the drug has been legalized, it has proven to be a valuable tool in managing chronic pain—in some cases working for patients for which conventional painkillers are ineffective.

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To determine exactly how cannabis relieves pain, a group of Oxford researchers used healthy volunteers, an MRI machine and doses of THC, the active ingredient in marijuana. Their findings, published today in the journal Pain, suggest something counterintuitive: that the drug doesn’t so much reduce pain as make the same level of pain more bearable.

“Cannabis does not seem to act like a conventional pain medicine,” Michael Lee, an Oxford neuroscientist and lead author of the paper, said in a statement. “Brain imaging shows little reduction in the brain regions that code for the sensation of pain, which is what we tend to see with drugs like opiates. Instead, cannabis appears to mainly affect the emotional reaction to pain in a highly variable way.”

As part of the study, Lee and colleagues recruited 12 healthy volunteers who said they’d never used marijuana before and gave each one either a THC tablet or a placebo. Then, to trigger a consistent level of pain, they rubbed a cream on the volunteers’ legs that included 1% capsaicin, the compound found that makes chili peppers spicy; in this case, it caused a burning sensation on the skin.

When the researchers asked each person to report both the intensity and the unpleasantness of the pain—in other words, how much it physically burned and how much this level of burning bothered them—they came to the surprising finding. “We found that with THC, on average people didn’t report any change in the burn, but the pain bothered them less,” Lee said.

This indicates that marijuana doesn’t function as a pain killer as much as a pain distracter: Objectively, levels of pain remain the same for someone under the influence of THC, but it simply bothers the person less. It’s difficult to draw especially broad conclusions from a study with a sample size of just 12 participants, but the results were still surprising.

Each of the participants was also put in an MRI machine—so the researchers could try to pinpoint which areas of the brain seemed to be involved in THC’s pain relieving processes—and the results backed up the theory. Changes in brain activity due to THC involved areas such as the anterior mid-cingulate cortex, believed to be involved in the emotional aspects of pain, rather than other areas implicated in the direct physical perception of it.

Additionally, the researchers found that THC’s effectiveness in reducing the unpleasantness of pain varied greatly between individuals—another characteristic that sets it apart from typical painkillers. For some participants, it made the capsaicin cream much less bothersome, while for others, it had little effect.

The MRI scans supported this observation, too: Those more affected by the THC demonstrated more brain activity connecting their right amydala and a part of the cortex known as the primary sensorimotor area. The researchers say that this finding could perhaps be used as a diagnostic tool, indicating for which patients THC could be most effective as a pain treatment medicine.