Ask a Doctor: What do scientific studies tell us about smoked marijuana?



Smoking pot to relieve pain and other symptoms is a contentious issue from the medical, ethical, regulatory and legal perspective. Many patients say, however, that pot works. But does it really work? Physicians and scientists need to know if there is truth to this (policy makers most often in my opinion, do not work with science but with people’s wishes, lobbies etc). But scientists can not just rely on what people say and must test the effects in properly designed studies that leave little room for misinterpretation.

Let’s look then at what kind of information currently exists from appropriately conducted scientific trials and reviews of scientific literature. I pulled out recent high quality studies published in respected medical journals, which attempt to answer the following questions: a) Do cannabinoids work for pain (ingested, inhaled, smoked or even given through injection)? 2) Does smoked cannabis in particular work for pain and if so, what kind of patients can benefit? 3) Does smoked cannabis work better than cannabis in the form of a pill? 4) Are there harms that outweigh the benefits of smoked cannabis?

  • An older systematic review (BMJ 2001;323:1-6) looked at 11 trials including a couple of hundred patients, mostly suffering from cancer pain, and a few with non cancer chronic pain and acute postoperative pain. Four different cannabinoids were tested (synthetic or semi-synthetic) but not smoked marijuana, to figure out whether cannabinoids in the first place work in human pain. The authors concluded that cannabinoids were no better than codeine (a weak opioid) in controlling chronic pain and had undesirable effects on the central nervous system.
  • A study published in Neurology (2007;68:515-521) in adult patients with neuropathy due to AIDS, concluded that “smoked cannabis was well tolerated and relieved pain as good as oral drugs for neuropathic pain”.
  • A study published in J. Pain (2008;9:6: 506-521) looked at patients with neuropathic pain from injury of the nerves, the brain or the spinal cord, and concluded that “smoked cannabis had good effect on pain, but impaired learning, memory and attention, particularly at higher doses of smoked THC”.
  • Another study (Neuropsychopharmacology 2009;34:672-680) studied patients with difficult-to-treat AIDS neuropathy and concluded that there was “a considerable reduction in pain when smoked cannabis was added in the pain medications they were already taking”.
  • A Canadian study (CMAJ 2010; 182(14):e694-701), in patients with neuropathic pain concluded that a “single puff of high concentration smoked THC three times a day for 5 days reduced pain, improved sleep, and was well tolerated”.
  • A systematic review of studies done including some form of cannabis, smoked or other (Pain Med 2009;10(8):1353-1368) concluded that “cannabis treatment of chronic pain has moderate beneficial effect, but this effect may be partially or completely offset by potentially serious harms”.
  • A study on patients with multiple sclerosis (CMAJ 2012;184(10):1143-1150) concluded that “smoked cannabis reduced spasticity and pain in this group of patients”.

Two more studies were done in normal people when they were submitted to an acute painful stimulus as part of an experiment.

  • A study published in Anesthesiology (2007;5:785-796) studied 15 volunteers who smoked marijuana cigarettes and then tested for pain after injecting a burning substance in their forearm. Acute pain was reduced in lower doses of smoked THC, but it increased significantly in higher doses of THC.
  • The latest study (Neuropsychopharmacology, advance online publication,15 May; 2013; doi:10.1038/npp.2013.97) tested normal people smoking pot daily by giving them an acute painful stimulus and comparing the effect of smoked marijuana and that of a marijuana pill (dronabinol). Both decreased pain, but the pill worked longer and it was less likely to be abused.


To summarize for the reader, this is what science says so far for pot effect:

  • Cannabis products in general and smoked cannabis in particular work for certain kinds of pain specifically due to injury of the nervous system (AIDS neuropathy, Multiple Sclerosis, Spinal Cord Injury etc);
  • The effect is no better than that of weak (but not strong) opioids or non opioid drugs for neuropathic pain;
  • Very low dose of smoked cannabis three times a day (a single puff actually each time) may work for pain and sleep problems in neuropathic pain patients;
  • Higher doses of cannabis have an adverse effect on memory and attention and can actually increase instead of decreasing pain;
  • When smoked cannabis is compared to cannabis in the form of a legally prescribed pill on experimental pain, the pill seems to have longer lasting effects.


All these trials were done for a very short period of time (days or few weeks). So, whatever the literature says, does not mean the effects can also apply to chronic users or those who use smoked pot for other kinds of pain.

In my next paper (and hopefully the last on the topic), I will review what scientific studies tell us about side effects and harms related to smoking pot.


Angela Mailis Gagnon, MD, MSc, FRCPC(PhysMed)

Director, Comprehensive Pain Program,

Senior Investigator, Krembil Neuroscience Centre

Toronto Western Hospital,

Chair ACTION Ontario