The pain relief experienced by marijuana users varies greatly among individuals, a brain imaging study at the University of Oxford suggests.
For some patients with chronic pain, cannabis or cannabis-based medications remain effective when other drugs have failed to control pain, while others report very little effect of the drug on their pain but experience side effects.
The researchers set out to investigate why some people experience strong relief while others do not.
“Cannabis does not seem to act like a conventional pain medicine. Some people respond really well, others not at all, or even poorly,” said lead researcher Dr. Michael Lee of Oxford University’s Centre for Functional Magnetic Resonance Imaging of the Brain (FMRIB).
“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.”
During the study, researchers found that an oral tablet of THC, the psychoactive ingredient in cannabis, tended to make the experience of pain more bearable, instead of actually reducing the intensity of the pain.
“We know little about cannabis and what aspects of pain it affects, or which people might see benefits over the side-effects or potential harms in the long term. We carried out this study to try and get at what is happening when someone experiences pain relief using cannabis,” said Lee.
The small-scale study in a controlled setting involved 12 healthy men and only one of many compounds that can be derived from cannabis. “That’s quite different from doing a study with patients,” Lee said.
“My view is the findings are of interest scientifically but it remains to see how they impact the debate about use of cannabis-based medicines. Understanding cannabis’ effects on clinical outcomes, or the quality of life of those suffering chronic pain, would need research in patients over long time periods,” he added.
For the study, the researchers conducted a series of MRI scans with each of the 12 volunteers at the FMRIB centre in Oxford.
Before a scan, patients were given either a 15 mg tablet of THC or a placebo. THC is the active psychotropic compound in marijuana – the ingredient that’s responsible for the high that drives recreational use of the drug.
To induce a certain level of pain, the volunteers also had a cream rubbed into the skin of one leg. This was either a placebo cream or a cream that contained 1 percent capsaicin, the ingredient of chilies that causes a burning and painful sensation.
Each participant was given four MRI tests to cover each combination of THC or placebo, and chili pain-inducing cream or placebo cream.
“The participants were asked to report the intensity and unpleasantness of the pain: how much it burned and how much it bothered them,” said Lee.
“We found that with THC, on average people didn’t report any change in the burn, but the pain bothered them less.”
Although this effect was statistically significant, there was great variability regarding THC’s effect on pain. Only six out of the 12 reported a clear change in how much the pain bothered them, for example.
The brain imaging results verified the reports of the participants. The change in unpleasantness of pain correlated with lowered activity in the part of the brain called the anterior mid-cingulate cortex. This structure is involved in many functions, and has previously been associated with the emotional aspects of pain.
‘We may in future be able to predict who will respond to cannabis, but we would need to do studies in patients with chronic pain over longer time periods,” Lee said.
The researchers report their findings in the journal Pain.
Source: University of Oxford