Is Weed Medicine? Here’s Where We’re at With the Evidence for Medicinal Cannabis Going Into 2020

Learn what the evidence says so far about medical cannabis

Pot, marijuana, reefer — whatever you call it, cannabis is the hot, new cure (some say) for everything from PTSD to cancer. It’s quickly being legalized in countries worldwide and, so far, in 33 US states. Yet there are still major gaps in our knowledge and evidence about its medicinal properties.

About 10% of the 22 million regular cannabis users in the United States use it for purely medical reasons, and another 36% of users report using cannabis at least sometimes for medical reasons. More than 1.2 million people are estimated to access medical cannabis through licensed dispensaries or licensed home cultivation in the United States (as of April 2018).

Policy around medical cannabis is created in part based on limited scientific research that shows that cannabis could be an effective treatment for some medical conditions. But deciding whether cannabis could be an effective treatment for your particular condition may be difficult. Reliable medical studies are difficult to access (the best medical journals often charge high prices for a subscription) and tough to understand (the studies are written by scientists for scientists). And with more studies coming out almost daily, new research sometimes contradicts previous work. 

So we’ve done the work for you. Here is your guide to what the best of the medical marijuana studies have found going into 2020. It’s intended for you to use as a reference for deciding whether cannabis could be an effective treatment for you. All medical decisions should be made in conjunction with a qualified healthcare provider.

Following are:  

What are cannabinoids?

Medical conditions and cannabis

Side effects of cannabis 

Is cannabis legal? 

What are cannabinoids? 

Cannabis contains a number of chemical compounds, called “cannabinoids”. These chemicals bind with cannabinoid receptors found throughout the human body, and it is this interaction that has a potentially beneficial medical, euphoric and other effects. Each cannabinoid may have different effects on the body., and so each has a different potential medical benefit.

The best known cannabinoid, THC (tetrahydrocannabinol), is mainly responsible for the psychoactive effects of cannabis. CBD (cannabidiol) is heavily marketed in a wide variety of products (creams, gels, infusions, edibles and more) as a non-psychoactive cannabinoid with purported health properties. But since it is relatively new, most of the claims still have little scientific support. CBD has been approved by the FDA for the management of 2 forms of pediatric epilepsy, Dravet syndrome and Lennox-Gastaut syndrome. Decades ago, in 1985, the FDA approved two other cannabinoids, dronabinol and nabilone, for use with nausea from chemotherapy and approved dronabinol for further use in diseases that cause dangerous weight loss, such as AIDS. 

Discussions about the medical utility of cannabis usually refer to one or several of these different cannabinoids.

Medical conditions and cannabis

People have claimed that cannabis may be a useful medical treatment for a number of health conditions. But very few have rigorous research evidence supporting their use—partly because there have been so many legal barriers to studying cannabis. And studies on the medical effects of marijuana also vary in terms of their methodological quality.

The medical studies on medical use of cannabis can be divided into four levels: the best is conclusive or substantial research support; then moderate research support; limited research support; and finally, insufficient research support. 

This list was created by first searching for research reviews about the medical effects of marijuana from the past 5 years, [1-7] with special attention to the 2017 National Academy of Sciences, Engineering, and Medicine’s (NASEM) comprehensive research review. Additional evidence for each of the conditions was included to fill in research gaps. The result is fairly comprehensive, but may not be an exhaustive list of the research on the medical effects of cannabis.

The best: conclusive or substantial research support.

The research has been fairly clear that marijuana is a useful medical treatment for several conditions. This category lists the conditions for which there is a substantial body of medium- to high-quality research that suggests cannabis can be effective for treatment and that the benefits outweigh the risks of known and unknown side effects..

Chemotherapy-induced nausea and vomiting. The FDA approved two cannabinoids (dronabinol and nabilone) for these conditions in 1985 based on high-quality evidence.Scientific consensus is that there is conclusive evidence that several oral cannabinoids are effective for chemotherapy-induced nausea and vomiting.[7,11] CBD has not been adequately tested for this use so it’s unclear at this time if it is effective. Similarly, the evidence on smoked marijuana is insufficient to say if it is useful.

Pain. There has been substantial evidence supporting the use of cannabinoids for managing non-cancer chronic pain, which has led the NASEM and others to conclude cannabinoids can effectively manage chronic pain.[4,7,19] Other research also suggests it can be effective for neuropathic pain from cancer, as well as palliative pain management.[4,6,11,19,21,22]

Epilepsy. The FDA has approved CBD for managing several forms of epilepsy in children (including Dravet syndrome and Lennox-Gastaut syndrome) based on several high-quality studies.[8,9]

Spasticity (from Multiple Sclerosis). Spasticity is a disorder in which a person has trouble controlling their muscles. There is substantial evidence that several cannabinoids can reduce symptoms of spasticity from multiple sclerosis.[1,4,7,11] However, there is insufficient evidence to support its use for spasticity that results from spinal cord injuries.[7,19]

Moderate research support.

Some fair-quality research suggests cannabis could be effective for the conditions in this category. While the research suggests cannabis may be useful, more is needed in order to be conclusive about its effectiveness, the risks of adverse events and the side effects for treating these conditions.

Anorexia and weight loss. The FDA approved dronabinol for stimulating appetite in patients with AIDS in 1992[1] and recent reviews have found support for this use, although the research is somewhat limited.[7,11] The research is insufficient to say whether cannabis is effective for the treatment of cancer-associated anorexia, or anorexia nervosa.[7,21]

Sleep disorders. The NASEM concluded that there is moderate evidence for the effectiveness of some cannabinoids (especially nabiximols) to improve sleep in those with sleep disturbances resulting from sleep apnea, multiple sclerosis, fibromyalgia, and chronic pain.[7,11]

Limited or weak research support.

For the following conditions, there has been some research evidence to suggest that various cannabinoids have a treatment effect. However, the research is either conflicting or limited in several important ways, which means that the findings are inconclusive. More research is needed to be confident in the effectiveness of cannabis for these conditions and to identify side effects and risk of adverse events..

Addiction. The NASEM included only three studies on the use of cannabis for individuals that wanted to abstain from addictive substances. Two of these were the use of cannabis for cannabis use disorder, and one was for people looking to quit smoking.[7] From these three studies, it was concluded that there was not enough evidence to determine the use of cannabis in reducing addiction.[13] However, more recent research suggests that cannabis is sometimes used as a substitute for pharmaceutical opioids [19] and may be useful in reducing addiction to opioids.[18,22] It appears there is some limited evidence of its effectiveness for opioid addiction.

Anxiety. One study of 24 patients with social anxiety disorder found that CBD resulted in improved anxiety.[16] Another found that patients reported that cannabis improved their anxiety symptoms.[27] However, given the lack of high-quality studies, researchers have concluded that the evidence for the use of cannabis to treat anxiety is limited.[7] CBD, in particular, looks promising as a treatment for anxiety, but there is not yet enough evidence to recommend its use.[2,7,11]

Diabetes. There is limited evidence that cannabis use is associated with a lower risk of diabetes and metabolic syndrome. However, there is also limited evidence of an association between cannabis use and a higher risk of prediabetes.[7]

Post-traumatic stress disorder (PTSD). There is strong anecdotal evidence for the effectiveness of cannabis to treat PTSD. Also, one study of ten10 male Canadian military personnel found that it decreased trauma-related nightmares.[15] However, the existing evidence is limited in several important ways [3,7], including for subgroups like children.[20]

Tourette’s syndrome. There is anecdotal evidence, supported by a few small studies, that suggests cannabis can reduce tics associated with Tourette’s syndrome.[5,11] However, the conclusion of scientists is that this evidence is not yet strong enough to support its use.[3,7,12,13]

Traumatic brain injury / Intracranial Hemorrhage. A few studies have suggested that the use of cannabis is associated with better outcomes from traumatic brain injury. These include reduced mortality and reduced disability. The NASEM concluded that there is limited evidence of the effectiveness of cannabis for brain injuries and intracranial hemorrhage.[7]

Insufficient research support.

For the following conditions, there is either an absence of, or insufficient, evidence to determine one way or another whether cannabis or individual cannabinoids have a treatment effect. It is possible that there is an effect, but this has not been established with the research we have now. There is also the possibility of side effects or adverse outcomes that have not been identified. 

Alzheimer’s. Several U.S. States have approved cannabis to treat agitation symptoms in people who suffer from Alzheimer’s. However, there is little research evidence supporting its effectiveness for this use.[3]

Amyotrophic lateral sclerosis (ALS). The NASEM found insufficient or no evidence to support the treatment effect of cannabis on this disorder.[7]

Cancer. There is some evidence from animal studies that cannabinoids can selectively kill cancerous tumor cells.[6] However, since the research on humans is very limited, the scientific conclusion is that there is so far insufficient research to say one way or another if cannabis has an effect on preventing or treating cancer.[6,7,13]

Depression. The NASEM found that there is some evidence that cannabis is not effective for the treatment of depressive symptoms in individuals with chronic pain or multiple sclerosis.[7] However, they did not find research on the effects for people with major depressive disorder. In other words, the effect of cannabis on people with depression is not yet clear.[11]

Dystonia (a neurological movement disorder). There is insufficient evidence to support (or refute) the effect of cannabis on treatment dystonia.[7]

Huntington’s disease. There is insufficient evidence to support the effect of cannabis on treatment of symptoms from Huntington’s disease.[7]

Irritable bowel syndrome. There is a lack of research suggesting that cannabis can help with IBSirritable bowel syndrome.[7]

Parkinson’s disease. The NASEM found insufficient evidence supporting the use of cannabis to help with the motor symptoms in patients who suffer from Parkinsons.[7]

Cannabis Is Not Effective for Some Conditions

Dementia. The NASEM concluded that there is some limited evidence that cannabis is not effective for the treatment of symptoms associated with dementia.[7]

Glaucoma. The NASEM concluded that there is some evidence, although it’s limited, that suggests cannabinoids are ineffective for the treatment of glaucoma.[7]

Side effects of cannabis

Research has looked as much into the potential adverse effects of cannabis use as it has the potential medical effects.[1] While there are side effects, it appears that for some medical uses the safety profile of cannabis is similar to, or even favorable to, alternative treatments.[6]

Vaping and e-cigarettes are sometimes used as a delivery for weed and cannabis derivatives. There is almost no research into the health benefits or risks of using pot in vapes or e-cigs. There are significant side effect risks associated with vaping liquid concentrates, especially those that contain vitamin e acetate, see MedShadow’s article: Are Vaping and E-Cigs Really So Bad for You?

There is evidence for the following side-effects

PregnancyAdverse perinatal outcomes. Cannabis use has been associated with some adverse perinatal outcomes, such as preterm births and lower birth weight.[1,7]

Bipolar symptoms. There is a possible link between very regular cannabis use (near-daily use) and greater bipolar symptoms in people that already have a bipolar disorder diagnosis.

Cancer. Current research suggests that using cannabis does not increase a person’s risk for several types of cancer (including lung cancer, or cancers of the head and neck). There is, however, limited evidence that cancer use is associated with one type of testicular cancer.

Cannabis use disorder. Cannabis use disorder is diagnosed when the pattern of use of cannabis is identified as problematic because it significantly affects the quality of a person’s work, school, or relationships.] Regular or chronic use of marijuana can lead to cannabis use disorder.[1,4,11] While it can be addictive, cannabis seems to be less addictive than heroin, alcohol, cocaine, and prescribed anxiety medication.

Cardiovascular disease. There is some worry that cannabis use may result in increased risk of cardiovascular conditions like heart attacks and stroke,[4,23] but the evidence is unclear.[7,11,23] Limited evidence suggests that cannabis may trigger heart attacks; however, cannabis use does not seem to be related to the risk of heart attack.[7,23] There is also some limited evidence that cannabis use is associated with the risk of having a stroke.[7,23]

Cognitive impairment and judgment. There is very good evidence that the use of cannabis can lead to short-term (up to 24 hours) cognitive impairment in learning, memory, attention, and judgment.[4,7,11] There is some evidence that this can be long-lasting for adolescents who use marijuana regularly.[1,4,11]

Injury and motor coordination. Use of marijuana has been associated with impaired motor coordination, and also with accidents, including motor vehicle accidents.[1,7,11]

Psychosocial effects. There is moderate evidence to show that cannabis use by adolescents is related to impairments of educational and academic achievement, employment and income outcomes, and social relationship outcomes.[4,7,11]

Psychotic disorders. There is substantial evidence that links the chronic use of cannabis with some psychotic disorders like schizophrenia.[4,7,11,14] This relationship looks like it could be causal, but that has not yet been established.[1,7] People who use a lot of cannabis are at the highest risk of developing schizophrenia.

Respiratory disease. While it appears that regularly smoking cannabis is associated with increased phlegm production and chronic coughing, there is no clear association with respiratory disorders like COPD, asthma, or worsening lung function.[7] Smoking cannabis does seem to be associated with increased incidence of bronchitis and respiratory infection.[4,11]

There is no clear evidence for the following side-effects.

Anxiety. Cannabis does not appear to increase the likelihood of developing most anxiety disorders, although there is some evidence it could increase the risk of developing social anxiety disorder.[7]

Depression. Cannabis does not appear to increase the likelihood of developing depression.[7]

Post-traumatic stress disorder. Cannabis does not appear to increase the likelihood of developing PTSD.[7]

Is Medical Cannabis Legal?

Some countries have very clear laws around cannabis. For example, Canada has legalized both recreational and medical cannabis use. New Zealand has legalized medical cannabis use but not recreational use.

In the United States, the legal status of marijuana is complicated and constantly changing. Marijuana is currently legal for medical use in 33 states and Washington D.C. Each state has different rules around how patients can be prescribed medical cannabis, and how it can be obtained. Here is a link to the National Council on State Legislatures’ Marijuana Overview for the latest on state legalization for recreational and non-prescription use. 

But even though medical cannabis is lawful in 33 states, it is still illegal at the federal level. Cannabis is listed as a Schedule 1 substance in the Controlled Substance Act.[17] This means that, the federal government does not consider, marijuana to have any accepted medical use or to be safe to use even under medical supervision. Schedule 1 drugs (including pot) are considered to have a high potential for abuse and dependency. Because of that federal laws prohibit individuals from possessing and using marijuana. It also means that physicians are prohibited from prescribing cannabis as medical treatment.[17]

So if it’s legal at the state level, but illegal at the federal level, is it legal? It’s not yet clear.

The Obama Administration avoided this issue by creating a policy where the federal government would not enforce the law against using cannabis in states where it was lawful for them to do so. In 2018, Attorney General Jeff Sessions reversed that policy and stated that federal prosecutors would now prosecute users of cannabis — even in states where it is legal. That means that all users, including those using medical cannabis consistently with the laws in their state, are at risk of arrest and prosecution.

In deciding whether cannabis could be an effective treatment for you, it is important to consider the potential legal risks of using it.

Source: National Conference of State Legislatures


There is very clear evidence that cannabis is a useful medical treatment for several medical conditions. But for many other conditions, the evidence is less clear. Research is also clear about many of the adverse side effects from using cannabis. If you are  evaluating whether cannabis could be an effective treatment for you, you and your health care provider should carefully consider these effects.

The question of legality is not a minor impediment. Federal law stands opposed to pot and it’s not clear if or when prosecutions for use of medical and medically prescribed marijuan might start. 


[1] Hill, K. P. (2019). Medical use of cannabis in 2019. JAMA, 322(10), 974-975.

[2] Rong, C., Lee, Y., Carmona, N. E., Cha, D. S., Ragguett, R. M., Rosenblat, J. D., … & McIntyre, R. S. (2017). Cannabidiol in medical marijuana: research vistas and potential opportunities. Pharmacological Research, 121, 213-218.

[3] Wilkinson, S. T., Radhakrishnan, R., & D’Souza, D. C. (2016). A systematic review of the evidence for medical marijuana in psychiatric indications. The Journal of Clinical Psychiatry, 77(8), 1050-1064.

[4] Hill, K. P. (2015). Medical marijuana for treatment of chronic pain and other medical and psychiatric problems: A clinical review. JAMA, 313(24), 2474-2483.

[5] Anavi-Goffer, S., Bloch, M., Budman, C., Coffey, B., Coffman, K., Jimenez-Shahed, J., … International Advisory Consortium on Medical cannabis/related drugs for Tourette and Tic Disorders (2019). Medical Marijuana Research: The Cannabis Consortium Review of the Literature. Tourette Association of America.

[6] Wilkie, G., Sakr, B., & Rizack, T. (2016). Medical marijuana use in oncology: A review. JAMA oncology, 2(5), 670-675.

[7] National Academies of Sciences, Engineering, and Medicine. (2017). The health effects of cannabis and cannabinoids: The current state of evidence and recommendations for research. National Academies Press.

[8] Devinsky, O., Cross, J. H., Laux, L., Marsh, E., Miller, I., Nabbout, R., … & Wright, S. (2017). Trial of cannabidiol for drug-resistant seizures in the Dravet syndrome. New England Journal of Medicine, 376(21), 2011-2020.

[9] Thiele, E. A., Marsh, E. D., French, J. A., Mazurkiewicz-Beldzinska, M., Benbadis, S. R., Joshi, C., … & Gunning, B. (2018). Cannabidiol in patients with seizures associated with Lennox-Gastaut syndrome (GWPCARE4): a randomised, double-blind, placebo-controlled phase 3 trial. The Lancet, 391(10125), 1085-1096.

[10] Deshpande, A., Mailis-Gagnon, A., Zoheiry, N., & Lakha, S. F. (2015). Efficacy and adverse effects of medical marijuana for chronic noncancer pain: Systematic review of randomized controlled trials. Canadian Family Physician, 61(8), e372-e381.

[11]Schrot, R. J., & Hubbard, J. R. (2016). Cannabinoids: Medical implications. Annals of Medicine, 48(3), 128-141.

[12]Wong, S. S., & Wilens, T. E. (2017). Medical cannabinoids in children and adolescents: A systematic review. Pediatrics, 140(5), e20171818.

[13]Abrams, D. I. (2018). The therapeutic effects of Cannabis and cannabinoids: An update from the National Academies of Sciences, Engineering and Medicine report. European Journal of Internal Medicine, 49, 7-11.

[14]Volkow, N. D., Baler, R. D., Compton, W. M., & Weiss, S. R. (2014). Adverse health effects of marijuana use. New England Journal of Medicine, 370(23), 2219-2227.

[15]Jetly, R., Heber, A., Fraser, G., & Boisvert, D. (2015). The efficacy of nabilone, a synthetic cannabinoid, in the treatment of PTSD-associated nightmares: A preliminary randomized, double-blind, placebo-controlled cross-over design study. Psychoneuroendocrinology, 51, 585-588.

[16]Whiting, P. F., Wolff, R. F., Deshpande, S., Di Nisio, M., Duffy, S., Hernandez, A. V., … & Schmidlkofer, S. (2015). Cannabinoids for medical use: A systematic review and meta-analysis. JAMA, 313(24), 2456-2473.

[17]Gostin, L. O., Hodge, J. G., & Wetter, S. A. (2018). Enforcing federal drug laws in states where medical marijuana is lawful. JAMA, 319(14), 1435-1436.

[18]Powell, D., Pacula, R. L., & Jacobson, M. (2018). Do medical marijuana laws reduce addictions and deaths related to pain killers? Journal of Health Economics, 58, 29-42.

[19]Allan, G. M., Ramji, J., Perry, D., Ton, J., Beahm, N. P., Crisp, N., … & Fleming, M. (2018). Simplified guideline for prescribing medical cannabinoids in primary care. Canadian Family Physician, 64(2), 111-120.

[20]Wong, S. S., & Wilens, T. E. (2017). Medical cannabinoids in children and adolescents: A systematic review. Pediatrics, 140(5), e20171818.

[21]Häuser, W., Fitzcharles, M. A., Radbruch, L., & Petzke, F. (2017). Cannabinoids in pain management and palliative medicine: An overview of systematic reviews and prospective observational studies. Deutsches Ärzteblatt International, 114(38), 627.

[22] Minnesota Department of Health. (2019). Medical cannabis study shows significant number of patients saw pain reduction of 30 percent or more. Retrieved from

[23]Pacher, P., Steffens, S., Haskó, G., Schindler, T. H., & Kunos, G. (2018). Cardiovascular effects of marijuana and synthetic cannabinoids: The good, the bad, and the ugly. Nature Reviews Cardiology, 15(3), 151.

[24]Lafaye, G., Karila, L., Blecha, L., & Benyamina, A. (2017). Cannabis, cannabinoids, and health. Dialogues in Clinical Neuroscience, 19(3), 309.

[25]Mackie, K. (2008). Cannabinoid receptors: Where they are and what they do. Journal of Neuroendocrinology, 20, 10-14.

[26]Dhopeshwarkar, A., & Mackie, K. (2014). CB2 Cannabinoid receptors as a therapeutic target—What does the future hold?. Molecular Pharmacology, 86(4), 430-437.

[27]Turna, J., Simpson, W., Patterson, B., Lucas, P., & Van Ameringen, M. (2019). Cannabis use behaviors and prevalence of anxiety and depressive symptoms in a cohort of Canadian medicinal cannabis users. Journal of Psychiatric Research, 111, 134-139.


Ramsay Lewis

Ramsay Lewis is a freelance writer, researcher, and analyst. He is interested in how research can be used to inform policy and personal health decisions. He writes for Crisp Text.

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