Chronic pain affects millions of people around the world. The cause of pain can be very hard or even impossible to detect, and it can be difficult to find an effective form of therapy. Even when the cause is clear, it can be a challenge to find a way to effectively combat ongoing pain.

For some patients who struggle with chronic pain, cannabis (also known as marijuana) and cannabis-based medications have been found to be effective. However, others find that it fails to reduce their pain, only bringing about the side effects associated with its use, which, according to the National Institute on Drug Abuse, include “distorted perceptions, impaired coordination, difficulty with thinking and problem solving, and disrupted learning and memory.” 

A new study by the University of Oxford, published in the journal Pain, aims to get a better look at what exactly occurs in the brain when someone uses cannabis as a therapy for pain relief. Researchers found that, although some people reported changes in their levels of pain, there appeared to be no significant changes in the parts of the brain that account for the experience of pain. It did, however, appear that cannabis affects the emotional response the patients to pain, but it does not do so in a consistent fashion.

The Expert Take

Dr. Michael Lee of the University of Oxford’s Centre for Functional Magnetic Resonance Imaging of the Brain (FMRIB), who was involved with the study, spoke with Healthline about how the study was conducted, as well as its implications for patients suffering from chronic pain.

Although the study was limited to a small group of men and examined only one possible THC compound, the study was intended to begin scratching the surface of what occurs in the brain during the use of THC (the psychoactive ingredient in cannabis).

“Patients may be prescribed multiple doses and for much longer periods of time to help them manage pain,” explains Lee. “We studied effects from a single dose of THC in healthy, drug-naïve volunteers for ethical and scientific reasons.”

Source and Method

The study involved 12 men in good health, who were each given a 15mg tablet of either THC or a placebo. THC, or tetrahydrocannabinol, is the ingredient in cannabis that produces a high, and accounts for the popularity of cannabis’ use as a recreational drug.

“THC is the main psychoactive compound of cannabis,” explains Lee. “It has been prescribed in the form of Marinol (US) or Dronabinol (Europe) and also as part of Sativex for chronic pain.”

In order to produce a mild amount of pain in these healthy individuals, one of two types of cream was rubbed into the skin of one leg. One type was a dummy cream that did not elicit pain, while the other contained one percent capsaicin, which causes a painful, burning sensation.

Magnetic resonance imaging (MRI) was conducted four times on each participant—once for each possible combination of cream, placebo, and THC. They were asked to explain to researchers the intensity of their pain, as well as how much it bothered them.

The Takeaway

It was not clear that cannabis had any actual effects on the parts of the brain that signal pain, but it did appear to aid some study participants in their emotional experience of that pain.

“Cannabis may influence the emotional aspect (‘the hurt’), rather than the sensation of pain,” explains Lee. “This sort of pain relief may depend on how the amygdala (the brain region linked to fear) reacts to the drug. That means that not everyone can benefit from the effects of cannabis on the brain." 

The researchers found an apparent correlation between the effectiveness of THC for pain relief and a certain connection in the brain—namely, the strength of the connection between the right amygdala and the primary sensorimotor area, which is part of the brain’s cortex. If this correlation is, in fact, present, it may allow doctors to determine in advance whether THC will be effective as a form of pain relief for certain patients. This will, however, require long-term studies, and would also require the participation of patients suffering from chronic pain.

Researchers determined that, although the burning sensation did not appear to change, THC did appear to make the pain less bothersome in some patients. However, out of the 12 participants, only six said there was an actual change in the degree to which their pain bothered them.

Other Research

2002 study in Pain Research & Management took a look at therapeutic cannabis use among patients experiencing chronic pain. Most of the patients surveyed in the study reported improvement in pain, mood, and sleep.

Future studies will need to examine the effects of THC over a longer period of time, and in patients who actually experience chronic pain, as opposed to the healthy men surveyed in the Oxford study.

“We do need longer-term studies to help predict which patients, or what sort of pain, might respond positively to cannabis-based medicines,” says Lee. “We also need to understand the long-term risks of cannabis in these patients.” 

Although the findings of the Oxford study will likely impact future research on THC and pain relief, it is unclear how the findings will affect the dialogue surrounding the use of cannabis. “The findings are of interest scientifically, but it remains to be seen how they impact the debate about use of cannabis-based medicines,” says Lee in a press release for the study.