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Home News Jury’s still out on cannabis treatment for mental illness
Nov 24, 2019 5 min read

Jury’s still out on cannabis treatment for mental illness

author
by Emily Earlenbaugh, PhD.
Medically reviewed by Roni Sharon, MD
Doctor writing medical cannabis prescription

As legal cannabis is becoming more available throughout the world, many are utilizing it to treat their own mental health conditions, such as depression, anxiety, PTSD, ADHD, psychosis, and Tourette’s syndrome. But is this justified by the medical research on cannabis? A new study, published in The Lancet Psychiatry journal, says not necessarily. 

While the authors did find some evidence that cannabis can help with certain types of mental illness, it was far from conclusive, and they also found evidence of some harmful effects it can have on certain mental disorders. All in all, they concluded felt that the evidence was too weak to justify the risk factors associated with cannabis use for mental illness. 

The study was an exhaustive meta-analysis on the last 40 years of studies on cannabis and mental health in human adults, 18 and over. The research was conducted by collaborators from the National Drug and Alcohol Research Centre at the University of New South Wales, Sydney; the Centre for Youth Substance Abuse Research at the University of Queensland, Brisbane; and the National Addiction Centre at Kings College London.

Still, despite the lack of strong evidence, researchers aren’t throwing away the idea that cannabis might be helpful for certain mental health conditions. The pool of research currently available simply lacks the kind of randomized and controlled studies that health professionals need to make firm conclusions about the effects cannabis might have on mental health. 

Furthermore, cannabis is an extremely varied plant with many genetic variations and different ways it might be taken and its impact varies from one individual to another. Clinical guidelines about how to properly treat various conditions cannot be formulated without that evidence.

Weak Evidence of Positive Effects on Mental Health

The study authors conducted this meta-analysis by searching through databases of past research to find trials conducted specifically on using cannabinoids for depression, anxiety, attention-deficit hyperactivity disorder (ADHD), Tourette syndrome, post-traumatic stress disorder (PTSD), and psychosis. They included studies on medical marijuana as well as pharmaceutical THC, CBD, or both. 

Researchers identified 83 studies that met their criteria, but few offered strong evidence of the positive effects of cannabinoids on mental health. In most cases, either the study design had limitations which made the data less reliable, or research simply failed to show statistical differences between cannabinoids and placebos. In addition, most of the research focused on pharmaceutical THC or a pharmaceutical blend of THC-CBD, (either of which might be cannabis-derived or synthetic), rather than the kind of raw medical cannabis that is being utilized by most cannabis patients.  

Still, from these studies, researchers did note that some of the evidence seemed to show benefits from cannabinoids on mental health. For example, in some studies, pharmaceutical THC-CBD led to significantly greater reductions in anxiety symptoms than did placebo. Still, the authors note that this evidence set is weak, in part because it hasn’t been widely replicated and in part, because these studies were looking at patients with multiple sclerosis and chronic pain, rather than those with anxiety disorders as the primary condition. Thus, it is possible that the reduction in anxiety was related to improvements in their other conditions rather than a direct effect on anxiety. 

No randomized controlled trials have looked directly at medical cannabis for treating anxiety disorders. But of the open-label studies conducted (studies where patients know what medication they have been given), the authors found one study looking at synthetic THC for anxiety specifically. This small study of only five participants did see significant reductions in anxiety symptoms. But with such a small group of participants, what we can draw from this research is very limited.

Some positive evidence was also found for a beneficial effect of cannabinoids on PTSD. The authors identified a single, small, randomized controlled trial of participants with post-traumatic stress disorder. This study found a significant benefit of pharmaceutical THC-CBD compared with placebo in improving global functioning and nightmare frequency in PTSD sufferers. Three open-label studies also found reductions in post-traumatic stress disorder symptoms for those using cannabinoids. 

In reviewing studies on cannabis use for psychosis, the authors found one controlled study that showed statistically significant global functioning improvements for patients with psychosis using CBD. They also note an open-label study that found CBD significantly reduced psychosis symptoms. 

While these studies suggested some improvements in mental health from using cannabinoids for these conditions, most studies on these conditions showed no statistically significant differences between cannabis and placebos for mental health. The authors were unable to identify any studies that showed significant improvements from using cannabinoids for depression, ADHD, and Tourette syndrome.

Evidence of Negative Impacts on Mental Health

In addition to finding some pieces of evidence that point to mental health improvements, the authors also note some reason to worry that cannabinoids could cause problems for mental health — particularly for those suffering from psychosis

A small randomized controlled trial looked at the use of pharmaceutical THC-CBD among those with psychosis. This study found no significant change in positive symptoms but did find a worsening of negative symptoms of psychosis and diminished cognitive functioning for those using these cannabinoids compared with a placebo. 

There is also some evidence that cannabis could negatively impact PTSD symptoms. While three open-label studies showed improvement in PTSD symptoms after cannabinoid use, one open-label study found that symptoms worsened with cannabis use in people with posttraumatic stress disorder and comorbid mental disorders.

The authors also note that the studied cannabinoids led to significantly more adverse events from treatment than the placebo during these studies. About one in every 14 participants withdrew due to adverse events. Still, this is a normal part of most drug studies, and the level of adverse events from cannabinoids did not differ significantly from competitors for treating the same mental health conditions. 

Beyond the risks emphasized in these particular studies, the authors also pointed to risk factors from other bodies of research on non-medical cannabis use, which were not a part of the review. These studies suggest that cannabis use can increase the occurrence of depression, anxiety, and psychotic symptoms. The authors note that this research includes a double-blind, randomized, placebo-controlled trial which indicates smoked cannabis increases the risk of acute psychotic symptoms. It also suggests that cannabis use for young adults (who are at the greatest risk of depression, anxiety, and psychosis) are at risk of developing dependence to cannabis if they use it daily over extended periods of time.

Researchers Advise Caution With Cannabis for Mental Health 

Given that we have some known risk factors for using Medical marijuana for mental health, and the current research supporting its benefits is weak, researchers on this meta-analysis advised caution for those seeking to treat a mental condition with cannabis. Saying that the risks and limitations of the evidence should be weighed when considering using cannabis for mental health, they warned that “those who decide to proceed should be carefully monitored for positive and negative mental health effects of using medicinal cannabinoids.” 

Still, researchers were careful to point out the constraints of their own analysis. The authors explain that their “analyses and conclusions are limited by the small amount of available data, small study sizes, and heterogeneity of findings across studies.”  

It is also important to note that most of the research focused on pharmaceutical THC or a pharmaceutical blend of THC-CBD, rather than specific whole-plant cannabis products, or high-CBD products, which are more commonly used by cannabis patients. When whole-plant cannabis was used, the type or chemical content was not specified. Since cannabis products can vary drastically in their effects, some of the heterogeneity in the research could be due to differences among the cannabis products used. 

In addition, many of the studies that provided evidence for these cannabinoids’ beneficial effects on conditions like anxiety, were actually studying patients with MS or chronic pain. The authors recommend that future research should, therefore, focus on the effectiveness of cannabinoids in patients diagnosed with these mental health conditions as their primary reason for treatment — particularly looking at those with anxiety and depression. 

“We need high-quality randomized controlled trials to properly assess the effectiveness and safety of medicinal cannabinoids, compared with placebo and standard treatments, for the treatment of mental disorders” the authors explain. But until that time, they recommend holding off on treating your mental health conditions with cannabis. 

“In light of the paucity of evidence and absence of good quality evidence, and the known risk of cannabinoids,” they explain, “the use of cannabinoids as treatments for mental disorders cannot be justified at this time.”

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