Cannabis 'safe to treat pain' but no proof it helps

"Smoking cannabis daily 'is safe when treating chronic pain – but only if you're an experienced user', study finds," says a Mail Online headline. It refers to a study done in Canada to see how safe medical cannabis is for treating chronic pain.

But the results of the study don't mean you should take cannabis if you have chronic pain. They don't show that cannabis helps to reduce pain, and the drug is also illegal in the UK.

In the study, more than 200 people with chronic (non-cancer) pain that had not got better with other treatments were given medical cannabis every day for one year. They were compared with a similar number of people who did not receive cannabis.

The study was designed to look at whether medical cannabis has any side effects – or adverse events – rather than its effect on pain. It found more non-serious adverse events in the cannabis group, but no difference between the two groups for more serious adverse events. It also found lung function test results for those who took cannabis changed very little over one year.

Cannabis use was associated with a small reduction in pain. But this wasn't the main outcome the study was looking at, and people weren't randomly allocated to the cannabis-taking and control groups.

This means it's not able to prove medical cannabis can reduce pain. Any small improvement would also have to be weighed against the increase in minor side effects shown by the study.

The findings are interesting and would benefit from further investigation with a large randomised controlled trial.

Chronic pain can be difficult to manage, and you may need to try different treatment options. Speak with your GP or health professional if you have chronic pain that isn't well controlled.  

Where did the story come from?

The study was carried out by researchers from a number of Canadian institutions, including McGill University, the Jewish General Hospital, and the University of British Columbia. Funding was provided by the Canadian Institutes of Health Research.

This study has not been reported widely. The body of the Mail Online article provides fair coverage, with a number of quotes from the researchers. However, the headline says "smoking cannabis", when only a quarter of the participants in the cannabis group chose to smoke it. Others used vaporisation or took it by mouth.

Also, it's not reliable to say the drug is "safe". People who took cannabis for pain experienced more adverse effects, albeit not serious ones. The study also can't tell us anything about the possible longer-term effects of medical cannabis on mental or physical health. 

What kind of research was this?

This was a prospective cohort study investigating the safety issues of people with chronic pain taking medical cannabis for pain management, compared with a control group of people who did not take cannabis.

A randomised controlled trial would be a better way to investigate this as findings are more likely to be a result of the intervention, rather than other factors or the natural course of the illness, which may have differed between the two groups.

What did the research involve?

Researchers assessed 431 adults from seven clinical centres across Canada who had been experiencing chronic moderate-to-severe non-cancer pain for six months or more. Participants had either not responded to conventional treatments or were considered medically inappropriate.

Potential participants were excluded if they:

  • were pregnant or breastfeeding
  • had a history of psychosis
  • had ischaemic (coronary) heart disease or arrhythmia
  • had lung disease

The intervention group included 215 people. Sixty-six percent of this group were current cannabis users, 27% were ex-cannabis users, and 7% had never used cannabis. 

This group took quality-controlled medical cannabis (12.5% tetrahydrocannabinol). It was taken in whichever way the participant felt the most comfortable – about a quarter smoked it; others used vaporisation or took it by mouth. An upper limit recommendation of 5g was issued (the average taken was 2.5g daily).

Thirty-two percent of the control group (216) were ex-cannabis users, while 68% had never used cannabis.

Adverse events (serious and non-serious) were the main outcome looked at by the researchers. Other outcomes examined were effects on brain (cognitive) function, which was assessed using various memory and intelligence tests, lung function and pain, which was measured on a scale from 1 to 10.  

Baseline assessments included addiction screening, neurocognitive testing, urine drug testing and, for the group taking cannabis, blood tests and lung function tests.

All participants were followed for one year, with the cannabis group receiving six clinical visits and three telephone interviews during that period. The control group had two clinical visits and five telephone interviews.

What were the basic results?

Baseline measurements showed the average pain intensity score at the start of the study was significantly higher in the cannabis group (6.6 out of 10) than the control group (6.1 out of 10). A higher number of control participants were using opioids (55% in cannabis group versus 66% in controls) and fewer were men (35% versus 51.2% of the cannabis group).

The rate of serious adverse events was not significantly different between the groups. A total of 13% of the cannabis group reported at least one serious adverse event, compared with 19% in the control group.

The most common serious adverse events in both groups related to the digestive system. For example, abdominal pain and intestinal obstruction each affected three people in the cannabis group. These serious adverse events were not considered related to cannabis use.

At least one non-serious adverse event was experienced by 88.4% in the cannabis group and 85.2% in the control group. However, the overall number of non-serious adverse events was significantly higher in the cannabis group (818) than the control group (581).

Non-serious adverse events that were very likely to have been related to cannabis use were:

  • drowsiness
  • amnesia
  • cough
  • nausea
  • dizziness
  • euphoric mood
  • excessive sweating (hyperhidrosis)
  • paranoia

Analysis of adverse events against previous cannabis use showed people with a history of cannabis use generally had fewer events overall.

There was no difference between the groups for cognitive results after one year and, in the cannabis group, lung function results showed no significant differences one year on.

The cannabis group saw a significant reduction in average pain intensity by 0.92 points over one year. Both groups saw an improvement in quality of life.

How did the researchers interpret the results?

The researchers concluded that: "This study evaluated the safety of cannabis use by patients with chronic pain over one year. The study found that there was a higher rate of adverse events among cannabis users compared to controls, but not for serious adverse events at an average dose of 2.5g herbal cannabis per day."

They go on to say the study cannot address the safety of medical cannabis for people who have never used the drug. Further studies are also needed to assess the long-term effects of medical cannabis on lung and cognitive functions beyond one year.


This prospective cohort study assessed the safety of medical cannabis for the management of chronic pain. It found a higher number of non-serious adverse events in people who took medical cannabis daily for pain.

Outcomes for serious adverse events and cognitive function were roughly the same as for people who did not take cannabis. Lung function results in the cannabis group remained unchanged over the course of the year-long study.

Though the study was not set up to examine effects on pain itself, it did find an improvement in those who used cannabis. However, this result should be interpreted with considerable caution.

This was not a randomised controlled trial where people were randomly allocated to cannabis use (or not) to balance out any differences between the groups. There may have been existing differences between people who did and did not use cannabis in terms of health and lifestyle factors, or differences in the type, quality and duration of pain.

This means this study is not able to prove medical cannabis can reduce pain. Also, it is difficult to know how meaningful a difference the pain improvement observed – less than 1 point change on a 10-point scale – would have made to individual people. Any small improvement would also have to be weighed against an increase in side effects.

Another limitation of this study is the large number of dropouts – 67 people taking cannabis and 34 controls – who left before the end of the study. Also, as the researchers acknowledge, though they found cannabis had no detrimental effect on brain or lung function, they did not examine this long term.

To summarise, the findings of this study are interesting and would benefit from further investigation in a large randomised controlled trial. However, for now the results do not suggest you should take cannabis if you have chronic pain. Nor does it confirm that cannabis is "safe". Cannabis is a class B drug that is illegal to possess or distribute. 

Medications that contain cannabinol – a non-psychoactive substances (e.g. it doesn’t get you ‘high’) extracted from cannabis - are legal as long as they have been given a licence by the Medicines and Healthcare products Agency (MHRA).

Chronic pain can be difficult to manage and different treatment options may need to be tried. Speak with your GP or the health professional looking after your care if you have poorly controlled chronic pain.

NHS Attribution