Research Review: Let's Study the Risks and Benefits of Medical Cannabis

Policy regarding the medical use of cannabis in the US is changing rapidly and differs dramatically depending on region. To date, thirty-three states and the District of Columbia have legalized cannabis for medical use, and many other states are working on legalization. Medical marijuana is the use of the marijuana plant or chemicals in it to treat diseases or conditions. Medical marijuana consists of the same chemical components as recreational marijuana, but it’s taken for medical purposes. Thousands of adults in the US are using cannabis to treat mental health issues such as anxiety and depression, yet, to date, there is little evidence from published studies showing that cannabis is an effective treatment for any mental health issue. This likely comes as a surprise to most people who logically assume that because a doctor will prescribe medical marijuana to them for anxiety or depression, there must be evidence to support that it works for these conditions. However, this is not the case currently. To be fair, lack of evidence may be related to difficulties in studying cannabis, which is currently illegal according to federal law. 

“Thousands of adults in the US are using cannabis to treat mental health issues …[yet] there is little evidence…showing that cannabis is an effective treatment for any mental health issue. “

People who choose to use cannabis for medical reasons should be making informed choices and not led by marketing or anecdotal reports. It is important to keep in mind that there are well-established relationships between regular cannabis use and a range of negative outcomes. Regular cannabis use has been shown to interfere with learning and memory (1), (2), affect brain structure (such as size of brain regions involved in memory and learning) (3), and lead to poorer mental health prognoses for people with depression, anxiety, and schizophrenia (4). These negative effects are worse in people who begin using cannabis at younger ages (5). Furthermore, regular cannabis use can lead to the development of misuse or addiction to cannabis (6), (7), particularly among daily users, 25-50% of whom will develop a cannabis use disorder (CUD) (8). Scientists and clinicians do not know whether individuals who are seeking cannabis as medicine will end up with unintended consequences, including addiction and difficulties with learning or memory. 

“People who choose to use cannabis for medical reasons should be making informed choices and not led by marketing or anecdotal reports.”

There is a clear need to study side effects of medical cannabis, which is not regulated by the Food and Drug Administration (FDA). An FDA drug review subjects medicines to safety evaluations, including a complete review of interactions with other drugs, side effects, and potential differences in effectiveness or safety among people of different genders, ages, and races. In contrast to medicines that undergo FDA review, medical cannabis, which was voted on at the ballot box, lacks information about the amount, or dose, people should take, safety of various medical marijuana products, effectiveness of various products for specific medical conditions, and side effects. How much medical cannabis is used, when, and how often, is largely left up to patients themselves. Little is known about the amount of cannabis needed to experience relief of symptoms, how long patients should use medical cannabis, or whether patients can substitute cannabis for other medications prescribed for their medical conditions. Perhaps most importantly, data is lacking on whether those who use medical cannabis daily for chronic conditions develop similar rates of addiction to those who use cannabis daily for recreational purposes.

Clinical trials, the gold standard in assessing whether a medicine works, have not examined side effects of medical cannabis such as cognitive effects or development of CUD. This is beginning to happen; for example, cannabidiol, or CBD, was approved by the FDA in 2016 in the form of a liquid called EPIDIOLEX® for the treatment of childhood epilepsy. EPIDIOLEX®, however, which is highly regulated and dosed, is quite different than medical cannabis purchased in a dispensary, which is unregulated. We may someday discover other therapeutic components of cannabis, which may eventually be useful for mental health treatment. However, until we have more evidence, we should take a cautious approach to using or recommending medical cannabis for mental health conditions. 


Jodi Gilman, PhD
Associate Professor, Harvard Medical School
MGH Center for Addiction Medicine


  1. Solowij, N., & Battisti, R. (2008). The chronic effects of cannabis on memory in humans: A review. Current Drug Abuse Reviews, 1(1), 81-98. doi: 10.2174/1874473710801010081
  2. Solowij, N., Stephens, R., Roffman, R.A., Babor, T. (2002). Does marijuana use cause long-term cognitive deficits? [author reply]. JAMA. 287(20), 2653-2654.
  3. Lorenzetti, V., Solowij, N., Fornito, A., Ian Lubman, D., & Yucel, M. (2014). The association between regular cannabis exposure and alterations of human brain morphology: an updated review of the literature. Current Pharmaceutical Design, 20(13), 2138-2167. doi: 10.2174/13816128113199990435
  4. Hall, W., & Degenhardt, L. (2009). Adverse health effects of non-medical cannabis use. The Lancet, 374(9698), 1383-1391.
  5. Hooper, S. R., Woolley, D., & De Bellis, M. D. (2014). Intellectual, neurocognitive, and academic achievement in abstinent adolescents with cannabis use disorder. Psychopharmacology, 231(8), 1467-1477. doi: 10.1007/s00213-014-3463-z
  6. Anthony JC. (2006) The epidemiology of cannabis dependence. In R.A. Roffman & R.S. Stephens (Eds.), Cannabis dependence: Its Nature, Consequences and Treatment (pp. 58-95). Cambridge University Press.
  7. Lopez-Quintero, C., de los Cobos, J. P., Hasin, D. S., Okuda, M., Wang, S., Grant, B. F., & Blanco, C. (2011). Probability and predictors of transition from first use to dependence on nicotine, alcohol, cannabis, and cocaine: Results of the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC). Drug and alcohol dependence, 115(1-2), 120-130. doi: 10.1016/j.drugalcdep.2010.11.004
  8. National Institute on Drug Abuse (2019, December). Marijuana DrugFacts