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Most states in the USA, and many countries across the world have passed legislation that allows the use of marijuana for medical purposes. Some of this legislation is dependent on various claims, many of which appear to be based on weak or nonexistent scientific evidence. Of all of the purported marijuana medical benefits, only a handful are supported by real evidence.

This review, Committee on the Health Effects of Marijuana: An Evidence Review and Research Agenda (pdf, which can be downloaded for free by registering or can be found online here), published by the influential and prestigious National Academies of Science, Engineering, and Medicine, examined more than ten thousand scientific studies that involved cannabis and various medical conditions. The value of such a review is that it examines not only the quantity of evidence supporting a claim but also the quality of such evidence. In the end, it gives much more weight to high-quality evidence.

I know many comments will drop on this article that “you haven’t read that incredible study published in Journal of Weed and Cancer Cures” – that misses the point. The National Academies is a highly respected institution, made up of the most respected scientists in the USA. And the committee that created this review is made up of leading public health, cancer, epidemiology, pharmacology, and psychiatry, all fields germane to understanding clinical and basic scientific research into cannabis.

Moreover, a review like this does two things – it gives more weight to well done clinical trials and pre-clinical studies, and it eliminates poorly done and biased studies. This is how science works – examine ALL of the evidence before coming to a conclusion. Pseudoscience, on the other hand, is to have a conclusion, like “weed cures cancer,” and only seeking evidence that supports that preordained conclusion.

Furthermore, and this cannot be stressed enough, this review is not opinion. It is not belief. It is not cherry picking. It is a critical analysis based on thousands of studies published in peer-reviewed journals. This is not published in a pro-cannabis website that cherry picks, misinterprets, and overrates a one-off study in an obscure journal. The report is over 400 pages long – most of you will not read even a few pages, because it is a dense scientific review written by some of the top scientists in the USA. Before you denigrate the study, I would suggest you read it carefully.

To save you time from reading the 400+ page opus, which I did, I divided up the medical evidence from strong to none of the evidence in support of benefits and of risks from smoking cannabis. Not to bury the lede, but there are only three conditions for which there is strong, overwhelming evidence benefits of marijuana. Just three.

Marijuana medical benefits

This is a huge report, so I’m going to look at various indications examined by the National Academy of Science scientists. They separated marijuana medical benefits for these indications into four categories – strong, moderate, limited, and no or insufficient evidence. We’ll look at them one by one.

Strong scientific evidence

There is strong scientific evidence supporting the following medical conditions:

For the treatment of chronic pain in adults (using all forms of cannabis)

As an anti-emetic (anti-nausea) in the treatment of chemotherapy-induced nausea and vomiting (in the form of oral cannabinoids)

For improving patient-reported multiple sclerosis spasticity symptoms (oral cannabinoids)

What’s important to note here is that smoking marijuana is only useful in treating chronic pain. For the other two indications, anti-nausea and multiple sclerosis, only the oral version of the drug, known as nabiximols, a highly specific extract from the cannabis plant, has shown efficacy. This contradicts a lot of the anecdotes and claims about the usefulness of marijuana in treating nausea and certain symptoms of multiple sclerosis.

This also supports the idea that the individual components of cannabis need to be identified, and the proper dosage of that component needs to be studied before we can make evidence-based claims about marijuana medical benefits. I’ll keep saying this – if a component of marijuana has a medical benefit, real research will determine what that is, isolate or synthesize it, put it into a form that safe for humans, and perform clinical trials and submit it to regulatory bodies, like the FDA, before marketing it for an indication. I know that sounds like a lot of hard work, but that’s how science-based medicine works.

Moderate scientific evidence

Next, there is moderate evidence supporting marijuana medical benefits for the following condition:

Improving short-term sleep outcomes in individuals with sleep disturbance associated with obstructive sleep apnea syndrome, fibromyalgia, chronic pain, and multiple sclerosis (using cannabinoids, primarily nabiximols)

Better cognitive performance among individuals with psychotic disorders and a history of cannabis use

Let’s be clear by what is meant by “moderate evidence.” According to the document, it is when “there is some evidence to support the conclusion that cannabis or cannabinoids are an effective or ineffective treatment for the health endpoint of interest.” It is important for the reader to note that moderate evidence can eventually be moved to “strong” or be set aside completely, depending on what further research tells us. Scientific conclusions are not static, they may change with new and better evidence.

Just to be clear, the reviewers used this statement to define “moderate evidence”:

For this level of evidence, there are several supportive findings from good- to fair-quality studies with very few or no credible opposing findings. A general conclusion can be made, but limitations, including chance, bias, and confounding factors, cannot be ruled out with reasonable confidence.

Also, note that the sleep outcomes, which is very interesting, is not from smoking or eating cannabis, it results from using well-studied marijuana-based cannabinoids. You cannot control the dosage from consuming marijuana, but drugs like nabiximols allow the physician to carefully control and monitor the dosage that provides the best outcomes for sleep.

Limited scientific evidence

Next, there is limited evidence supporting marijuana medical benefits for the following medical conditions:

Increasing appetite and decreasing weight loss associated with HIV/AIDS (all forms of cannabis especially oral cannabinoids)

Improving clinician-measured multiple sclerosis spasticity symptoms (oral cannabinoids)

Improving symptoms of Tourette syndrome (THC capsules)

Improving anxiety symptoms, as assessed by a public speaking test, in individuals with social anxiety disorders (cannabidiol, one of the major active ingredients in cannabis)

Improving symptoms of post-traumatic stress disorder (using nabilone; a single, small fair-quality trial; more studies are required to fully accept this benefit)

Better outcomes (i.e., mortality, disability) after a traumatic brain injury or intracranial hemorrhage

Improving symptoms associated with dementia (cannabinoids)

Improving intraocular pressure associated with glaucoma (cannabinoids)

Reducing depressive symptoms in individuals with chronic pain or multiple sclerosis (nabiximols, dronabinol, and nabilone)

According to the report, limited evidence means:

For this level of evidence, there are supportive findings from fair-quality studies or mixed findings with most favoring one conclusion. A conclusion can be made, but there is significant uncertainty due to chance, bias, and confounding factors.

In the world of evidence-based medicine, this means that there is insufficient evidence to use cannabis (or its components) to treat any of these conditions. It certainly does not provide sufficient evidence to replace standard treatments with marijuana.

No or insufficient scientific evidence

Finally, there is no or insufficient evidence to support marijuana medical benefits for treating any of the following conditions:

Cancers, including glioma (cannabinoids)

Cancer-associated anorexia-cachexia syndrome and anorexia nervosa (cannabinoids)

Symptoms of irritable bowel syndrome (dronabinol)

Epilepsy (cannabinoids)

Spasticity in patients with paralysis due to spinal cord injury (cannabinoids)

Symptoms associated with amyotrophic lateral sclerosis (cannabinoids)

Chorea and certain neuropsychiatric symptoms associated with Huntington’s disease (oral cannabinoids)

Motor system symptoms associated with Parkinson’s disease or the levodopa-induced dyskinesia (cannabinoids)

Dystonia (nabilone and dronabinol)

Achieving abstinence in the use of addictive substances (cannabinoids)

Mental health outcomes in individuals with schizophrenia or schizophreniform psychosis (cannabidiol)

Please note carefully that there is no evidence that marijuana can treat or prevent any cancer – considering there are more than 200 different cancers, each with different causes, development, and treatment, it is biologically implausible to assume that marijuana would treat them all. We constantly read claims about marijuana medical benefits with regards to many (or all) of these conditions, yet there is no robust or even weak evidence in support of the clinical use of marijuana or any of its components.

Marijuana risks

In medicine, there is an important calculation that has to be made – do the benefits of a medical therapy outweigh the risks? If you break your leg, there are some significant risks from reducing the fracture so that you can walk again – some of those risks include death from a fat embolism. However, I bet that no one has ever refused consent to a physician to set that bone, because the benefits, walking normally, far outweigh the risks. So, if we are going to examine marijuana medical benefits, we also have to examine the risks from use, especially smoking it.

According to the report, there is strong evidence of an association between cannabis use and these medical conditions:

Worsens respiratory problems, such as chronic bronchitis episodes from long-term cannabis smoking

Increased risk of motor vehicle accidents

Low birth weight in babies

The development of schizophrenia or other psychoses, with an increased risk among the most frequent users

Next, there is moderate evidence of an association between marijuana and:

Increased risk of overdose injuries, including respiratory distress, among pediatric populations

Increased symptoms of mania and hypomania in individuals diagnosed with bipolar disorders (regular cannabis use)

A small increased risk for the development of depressive disorders

Increased incidence of suicidal ideation and suicide attempts with a higher incidence among heavier users

Increased incidence of suicide completion

Increased incidence of social anxiety disorder (regular cannabis use)

The development of substance dependence and/or a substance abuse disorder for substances, including alcohol, tobacco, and other illicit drugs

Worsening of negative symptoms of schizophrenia among individuals with psychotic disorders

Finally, there is limited or no evidence of an association between cannabis and:

The incidence of testicular germ cell tumors, esophageal cancer, prostate cancer, cervical cancer, glioma, non-Hodgkin lymphoma, penile cancer, anal cancer, Kaposi’s sarcoma, bladder cancer, acute myeloid leukemia/acute non-lymphoblastic leukemia, acute lymphoblastic leukemia, rhabdomyosarcoma, astrocytoma, or neuroblastoma in offspring. Moreover, there was moderate of no statistical association between cannabis use and incidence of lung, head and neck cancers.

Impaired academic performance, educational attainment, or social engagement

Heart attack, stroke, diabetes, anxiety, and bipolar episodes in people not diagnosed with bipolar disorder

Asthma or other chronic obstructive pulmonary diseases (COPD) when controlled for tobacco use

Death due to cannabis overdose

Pregnancy complications for the mother

Admission of the infant to a neonatal intensive care unit

Later outcomes in the offspring (for example, sudden infant death syndrome, cognition/academic achievement, and later substance abuse)

An increase in positive symptoms of schizophrenia (e.g., hallucinations) among individuals with psychotic disorders

The likelihood of developing bipolar disorder, particularly among regular or daily users

The development of any type of anxiety disorder, except social anxiety disorder

Increased symptoms of anxiety (near daily cannabis use)

What I find interesting is that there are substantial psychiatric risks to using cannabis, especially with long-term usage. For example, there appears to be powerful evidence for increased risk of the development of psychotic behavior, mania or hypomania, and depression in those individuals who were previously diagnosed with various disorders – these are serious mental health issues that need to be a part of the conversation about the risks of marijuana use, especially with mental health patients. These issues should not be dismissed by marijuana advocates, as if they are meaningless.

On one had, it seems that marijuana lacks any link to various cancers, cardiovascular disease, pulmonary diseases, and other issues. On the other hand, I think there are legitimate, and biologically plausible, concerns that long-term inhaling of any environmental pollutants, like marijuana smoke, can be harmful. For example, there is good evidence that second-hand marijuana smoke may increase risks for cardiovascular events.

Marijuana medical benefits – TL;DR

In summary, a committee of 16 highly qualified scientists closely examined the evidence and found precisely three benefits of marijuana in medicine supported by strong evidence – treatment of chronic pain, treatment of nausea, and improving spasticity symptoms of multiple sclerosis. That’s it. Yes, there is some moderate evidence for a positive benefit a few other medical symptoms, but the best medical evidence only supports three.

Furthermore, despite some of the more outrageous claims from the supporters of marijuana, there is just no evidence, weak or otherwise, that marijuana has any benefit to the treatment or prevention of any of the 200 or so cancers, which, as you might notice, are claims that bother me the most.

And we cannot set aside the significant mental and medical risks of using cannabis. Do they outweigh benefits? For some of the benefits that are scientifically supported, it’s a decision between a trained physician and the patient. Many of the conditions in which evidence shows strong or moderate support for marijuana can be more effectively and safely treated with current medical therapies.

I know what will be written in the comments – I’m a shill for Big Pharma that’s suppressing an easy cure for cancer. Or some website proves I’m wrong. Or the National Academy of Sciences is a bunch of anti-marijuana liars. Or we ignore that 1978 mouse study, never repeated, that showed that marijuana cures baldness. Or whatever.

However, I have no opinion about marijuana medical benefits, I only care about scientific evidence. I have a bias towards high-quality evidence in high quantities. This report examined thousands of studies (I doubt marijuana advocates have read even 1 or 2), and, using the scientific method, they arrived at solid conclusions. If you’re going to argue that these respected scientists have no clue based on your opinion, well that’s laughable.

Marijuana has a lot of potential for some medical indications – but a lot of tropes about it being pushed these days is just not supported by real medical evidence. And please, stop with the “marijuana cures cancer” nonsense. There is no evidence.

Note – this article was first published in June 2017. It required some copyediting, re-formatting, and cleaning up. Also, I fixed a couple of broken links and added some new links.

Citations

National Academies of Sciences, Engineering, and Medicine; Health and Medicine Division; Board on Population Health and Public Health Practice; Committee on the Health Effects of Marijuana: An Evidence Review and Research Agenda. The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research. Washington (DC): National Academies Press (US); 2017 Jan 12. doi: 10.17226/24625.

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