Weeding through the evidence on medical cannabis

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By Dr. Janice Mann

We call this column Evidence Matters. The reason is pretty simple. Evidence really does matter when making an important health decision. Often, when those in health care need to make decisions but have questions about a drug, medical device, test, or procedure, they turn to CADTH — an independent agency that finds, assesses, and summarizes the research on drugs and medical devices.

On August 24, 2016, Health Canada announced the Access to Cannabis for Medical Purposes Regulations, which allow Canadians access to a reasonable amount of cannabis prescribed by health care practitioners for medical purposes. Conditions treated with cannabis include nausea and vomiting, loss of appetite, and pain. Patients, clinicians, and others in the Canadian health care system have been asking for evidence on the medical use of cannabis to help guide treatment decisions. Below are just a few of the questions that have been posed to CADTH — and the answers we were able to find.

Does cannabis work to treat chronic pain?

For one evidence review on this topic we were asked to look specifically at nabilone — a synthetic version of cannabis — to see if it works to treat chronic pain. There wasn’t a lot of evidence to help answer this question and there were some limitations to it — but the evidence we did find shows some positive benefits and limited harms when nabilone is used to treat chronic pain. As is often the case though, more research is needed to be able to definitively answer this question.

For a second review of the evidence on this topic, we were asked to look specifically at a cannabis-based spray to see if it works and if it’s safe to treat neuropathic pain or other types of chronic pain. It’s a combination of two products, delta-9-tetrahydrocannabinol and cannabidiol (THC:CBD) marketed under the name Sativex®, and is taken by spraying it under the tongue or inside the cheek. Five systematic reviews were found, and they show that the spray may lead to favourable outcomes for patients in the short term, including reduced pain, and is well tolerated when compared with a placebo (no active medication). However, whether treatment of pain with the cannabis-based spray is beneficial and safe over the long term is uncertain. And there was no evidence comparing the spray to other pain treatments, so it’s not clear how well it works compared to other medications. One evidence-based guideline does recommend TCH:CBD spray as a third-line option for the treatment of neuropathic pain when other treatments are not adequate.

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Does cannabis work to treat other conditions, like post-traumatic stress disorder (PTSD)?

CADTH has been asked to look at the evidence to answer this question a number of times since 2009. Our most recent review looked at how well both cannabis and synthetic versions work to treat PTSD and whether there were any clinical practice guidelines to guide their use. One systematic review was found that included six individual studies. The included studies weren’t considered high-quality but they did find evidence to support the effectiveness of smoked marijuana, oral THC, and nabilone in reducing some symptoms of PTSD. Side effects were covered in only one of the studies on nabilone and were reported to be mild to moderate. No guidelines offering recommendations on the use of cannabis to treat PTSD were found.

Is cannabis safe to use with other medications? Does cannabis interact with other medications, drugs, or alcohol?

This is an important question but unfortunately there isn’t a lot of evidence to help with an answer. In our review of the evidence, one systematic review shows that nabilone may decrease the need for other medications such as opioids, NSAIDs, tricyclic antidepressants, dexamethasone, and ondansetron when used together, but it may make the effect of diazepam (a drug that depresses the central nervous system) stronger when taken together with codeine and alcohol. This isn’t much evidence and whether it’s high quality evidence isn’t clear, so it isn’t a lot to go on to guide decisions. But this is an area we’re likely to see more evidence on in the near future as medical use of cannabis increases.

Can cannabis help in the treatment of addictions?

Although cannabis may itself be addictive, it may also have anti-addictive properties that could help to treat addiction. In one CADTH review of the evidence, we looked to see if cannabis works to help treat addiction in residential transition or addiction programs. And despite our search efforts, we did not find any evidence to help answer this question. The role of cannabis in treating addiction remains uncertain.

These are just a few of the evidence reviews that CADTH has been asked to do on topics related to medical cannabis. You can find all of our related evidence at www.cadth.ca/cannabis. But CADTH is just one organization of many working to answer your questions about the medical use of cannabis. The Canadian Centre on Substance Use and Addiction (CCSA) has a focus on marijuana research including the Clearing the Smoke on Cannabis series. Health Canada also has many resources on the medical use of cannabis, and these and other resources can be accessed directly or through the additional resources section of our cannabis evidence bundle.

If you’d like to learn more about CADTH and our evidence reviews on this and other topics, please visit: www.cadth.ca, follow us on Twitter at @CADTH_ACMTS, or speak to the CADTH Liaison Officer in your region.

Dr. Janice Mann is a knowledge mobilization officer at CADTH.