Medical marijuana helps make pain more tolerable, but it does not reduce the intensity of pain, according to a small study at the University of Oxford.
Researchers studied the brain scans of a dozen men given an oral tablet of THC, the psychoactive ingredient in cannabis, and found reduced activity in key areas of the brain that substantiated their pain relief.
“We have revealed new information about the neural basis of cannabis-induced pain relief,’ said Dr. Michael Lee of Oxford University’s Centre for Functional Magnetic Resonance Imaging of the Brain. “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.”
Lee and his colleagues carried out a series of magnetic resonance imaging (MRI) scans on 12 healthy men. Before each scan, the men were given either a 15mg tablet of THC or a placebo. THC, or delta-9-tetrahydrocannabinol, is the active compound in marijuana – the ingredient that’s responsible for the “high” that drives recreational use of the drug.
To induce a level of pain, the men also had a cream rubbed into the skin of one leg. This was either a placebo cream or a cream that contained 1% capsaicin, the ingredient in hot peppers that causes a hot, burning and painful sensation.
The men were asked to report the intensity of their pain, how much it burned and how much it bothered them.
“We found that with THC, on average people didn’t report any change in the burn, but the pain bothered them less,” said Lee, who notes there was great variability in the men’s reactions. Only six out of the 12 reported a clear change in how much the pain bothered them.
The MRI scans substantiated the men’s reactions. Changes in the unpleasantness of pain were matched with a suppression of activity in the part of the brain called the anterior mid-cingulate cortex. This structure sits in a deep part of the brain and has been linked to the emotional aspects of pain. There were also changes in the right amygdala of the brain, which can be “primed” by chronic pain.
Of most interest to the researchers, was the strength of the connection between the right amydala and a part of the cortex called the primary sensorimotor. The strength of the connection correlated with the effects that THC had on the men’s pain. This suggests there might be a way to predict who would see benefits from taking cannabis for pain relief.
“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,” Lee said.
“Cannabis does not seem to act like a conventional pain medicine. 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.”
Lee adds that any kind of pain relief, whether emotional or physical, has value.
“It is impossible to recommend one single treatment, especially when there are so few therapies for intractable pain. It needs a combined approach that can involve painkillers, exercise and physiotherapy, counseling, even surgery,” said Lee.
“Living with chronic pain involves more than just coping with the pain. It runs through your life, through your decisions and what you’re motivated to do. Some people might have a known cause for the pain – MS, cancer or an injury – but many others may have no specific cause or diagnosis for their lower back or neck pain, abdominal pain, headaches or other symptoms. That can be very difficult to accept and it can be difficult to treat.”
Lee’s findings are reported in the journal Pain. The study was funded by the UK Medical Research Council and the National Institute for Health Research. Women were not included in the study because their menstrual cycles may have influenced levels of pain over the course of the 4–6 week study.