Marijuana works to relieve pain in some individuals by making users not notice the hurt so much, according to a landmark new study published by the scientific journal Pain.

Researchers at the Oxford Biomedical Research Centre took MRI scans of brain activity in people who were given an oral tablet of THC, then compared them to a control group who’d been given a placebo.

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Once inside the MRI, research staff rubbed a cream containing capsaicin onto one leg for each subject, inducing a burning sensation that triggered noticeable brain activity.

“The participants were asked to report the intensity and unpleasantness of the pain: how much it burned and how much it bothered them,” study author Dr. Michael Lee said in an advisory. “We found that with THC, on average people didn’t report any change in the burn, but the pain bothered them less.”

The subjects who took THC did not, however, exhibit a consistent response to the drug, with only six out of 12 saying they felt some relief.

Scans revealed that as the pain increased, other parts of the brain made adjustments to alleviate the worst of the sensation. Researchers also noticed that individuals whose brains exhibited a strong connection between the primary sensorimotor area and the right amygdala may respond more strongly to pain relief induced by marijuana.

“Cannabis does not seem to act like a conventional pain medicine,” Lee said. “Some people respond really well, others not at all, or even poorly. 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.”

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