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.

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.