An analysis of existing reports on the effectiveness of cannabis for treating pain and post-traumatic stress disorder finds no high-quality evidence in its defense.
This was the conclusion of two systematic reviews conducted by the United States Department for Veteran Affairs and published in the Annals of Internal Medicine.
The use of cannabis, or marijuana, for medicinal purposes - such as in the management of chronic pain and post-traumatic stress disorder (PTSD) - is becoming more accepted, both in the U.S. and globally.
Most U.S. states have now legalized use of cannabis in some form. Of these, eight states and the District of Columbia have also legalized it for recreational as well as medicinal use.
Cannabis and management of pain
The review authors note that, in the U.S., up to 4 out of 5 people who seek medicinal cannabis use it to manage pain.
Also, among patients receiving long-term prescription opioid therapy for pain, as many as 39 percent are likely to be using cannabis as well.
"However," note the authors, "little comprehensive and critically appraised information exists about the benefits and harms of using cannabis to treat chronic pain."
In their systematic review, they examined data from 27 chronic pain trials and found only "low-strength" evidence that cannabis relieves neuropathic pain and "insufficient evidence" that it alleviates other types of pain.
The authors also found some limited evidence on the harms of cannabis use in the general population, suggesting that cannabis use may raise the risk of psychotic symptoms, short-term cognitive impairment, and motor vehicle accidents.
There also appears to be insufficient evidence about the long-term physical harms in both heavy and long-term cannabis users, or in older populations.
Expert comments on discrepancy with use
In an accompanying editorial, Sachin Patel, an associate professor at Vanderbilt Kennedy Center in Nashville, TN - where he researches the effect of cannabis compounds on the brain in psychiatric disorders - comments on the finding.
He draws attention to the discrepancy between this result and the fact that pain is one of the most common reasons that people seek use of medicinal cannabis, and why many states have approved it for such use.
One reason could be that most of the studies used lower doses or strengths of cannabis compounds than those available in dispensaries, and the effect on pain depends on dose.
Another reason could be that the people who use medical cannabis for pain relief may not always be represented in clinical studies.
He also suggests that there could a "more controversial" explanation. Pain is a complex mix of "behavioral, emotional, and cognitive domains," the effects of which are not readily captured in traditional rating scales.
Prof. Patel leaves the issue undecided. On the one hand, there appears to be a need for a "more comprehensive analysis of patient well-being and functioning than is measured by common pain scales" to detect the effects of cannabis.
He notes that on the other hand, it may be possible that the effects of cannabis on perceived pain are "simply not robust," and that using a catch-all diagnosis of "pain" to justify legalizing medicinal cannabis "may be overused."
Cannabis and PTSD
Over a third of patients who request medicinal use of cannabis in states where it is legal give PTSD as the main reason.
However, as in the case of pain management, the authors state that the effectiveness of cannabis "in treating PTSD symptoms remains uncertain."
The team examined data from reviews, trials, and other studies that included a control group and reported benefits and harms of giving adults plant-based (as opposed to synthetic versions) cannabis to treat PTSD.
They found insufficient evidence to support the idea that cannabis relieves symptoms of PTSD.
One of the studies they reviewed - which was one of the largest to follow veterans with PTSD - did, however, show evidence that symptoms got worse in those veterans who continued or started to use cannabis compared with those who had never used it or who stopped using it during the study.
In his editorial, Prof. Patel says that the two reviews "largely echo" findings of organizations such as the National Academies of Sciences, Engineering, and Medicine, and that they suggest a "growing consensus in the field."
He suggests that even if further research reveals that there is no clear, substantial evidence that cannabis is an effective and safe treatment for pain or PTSD, it is unlikely that legislation will "remove these conditions from the lists of indications for medical cannabis."
"It will be up to front-line practicing physicians to learn about the harms and benefits of cannabis, educate their patients on these topics, and make evidence-based recommendations about using cannabis and related products for various health conditions."
Footnote: Marijuana or cannabis?
A quick note on terminology: marijuana or cannabis? The World Health Organization (WHO) define cannabis as a generic term for the several psychoactive substances that are present in the plant Cannabis sativa.
In reference to marijuana, the WHO say that in many countries, the term refers to cannabis leaves or other parts of the plant.
In the two reviews covered in this article, the researchers analyzed evidence on "plant-based cannabis preparations or whole-plant extracts." Because they used the term cannabis, so have we.