Is Weed Medicine? Evidence for Medical Cannabis

Learn what the evidence says so far about medical cannabis
medical marijuana evidence
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Pot, marijuana, reefer — whatever you call it, medical 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 37 US states. Yet there are still major gaps in our knowledge and in the evidence about its medicinal properties.

About 10% of the 32 million regular cannabis users in the United States use it for purely medical reasons, and another 36% report using cannabis at least sometimes for medical reasons. 

Policy around medical cannabis is created in part based on limited research that shows that it 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 through the year 2022. This article is intended to be used as a reference on medical cannabis and cannabis-based medications. All medical decisions should be made in conjunction with a qualified healthcare provider.

What are cannabinoids and cannabis-based medications? 

Cannabis contains a number of chemical compounds called “cannabinoids”. These chemicals bind with cannabinoid receptors found throughout the human body, and it is these interactions that have potentially therapeutic effects.

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, edibles and more) as a non-psychoactive cannabinoid with purported health properties. But since CBD extracts are relatively new, most of the health claims still have little scientific support. One exception is that a prescription formulation of CBD has been approved by the Food and Drug Administration (FDA) for the management of pediatric epilepsy.  Discussions about the medical utility of cannabis refer to THC, CBD, or combinations of the two. Cannabis-based medications contain either specific quantities of THC and/or CBD extracted from plant cannabis, or synthesized versions that are chemically identical to THC or have the same effects once metabolized. Whole plant cannabis contains both THC and CBD with the relative content depending on the botanical strain.

Medical conditions and  medical cannabis

There are widespread claims made by the cannabis industry as well as independent organizations that cannabis is a useful medical treatment. But very few of these claims are supported by rigorous research evidence—partly because there have been so many legal barriers to studying cannabis. Studies on the medical effects of marijuana also vary in terms of their methodological quality.

The research on the therapeutic benefits of cannabis can be divided into four levels: some treatment claims are supported by conclusive or substantial research; some treatment claims have moderate research support; some have only limited research support; and finally, for many treatment claims there is insufficient research support.

Conclusive or substantial research support.

This category lists the conditions for which there is a substantial body of quality research that suggests cannabis can be effective for treatment. However, for most of these conditions (with the exception of chronic pain) cannabis is not the most effective therapy available and is not used as a first-line treatment. Cannabis-based medications are typically used in conjunction with other medications or when other treatments have failed or produced side effects.

Chemotherapy-induced nausea and vomiting. The FDA approved two THC-based medications (dronabinol and nabilone) for these conditions in 1985 based on high-quality evidence. Scientific consensus is that there is conclusive evidence that oral THC is effective for chemotherapy-induced nausea and vomiting. Typically these medications are used in conjunction with other medications for nausea. CBD has not been adequately tested for this use so it’s unclear at this time if it is effective. However, a clinical trial is underway to determine if a combination of THC and CBD is more effective or better tolerated than THC alone. 

Chronic Pain. There is substantial evidence supporting the use of cannabinoids for non-cancer chronic pain, which led the National Academies of Sciences, Engineering, and Medicine (NASEM) to conclude in their 2017 report that cannabis-based medications can contribute to pain management. A more recent review specifies that the evidence is strongest for three medications; nabilone, dronabinol, and nabiximols. Though they are not effective for every individual, the THC-based medications nabilone and dronabinol have been shown to reduce pain by an average of one to one and a half points on a ten-point scale compared to placebo. Nabiximols contains equal parts THC and CBD rather than THC alone. It is slightly less effective but may be better tolerated than the THC-only medications; however it is currently not available in the US. All three of these medications are associated with side effects, notably sedation and dizziness. Few options are available for effectively managing chronic pain, and patients are increasingly favoring cannabis-based medications as an alternative to opioids.

Epilepsy. The FDA has approved the purified CBD medication Epidiolex for managing several forms of epilepsy in children (including Dravet syndrome and Lennox-Gastaut syndrome) based on several high-quality studies. Epidiolex is used as an add-on treatment for patients who are already taking other anticonvulsant medications; it has not been evaluated as a standalone treatment.

Spasticity (from multiple sclerosis). Spasticity is a symptom of multiple sclerosis in which a person has trouble controlling their muscles – it also occurs in other disorders. There is substantial evidence that cannabinoids can reduce symptoms of spasticity from multiple sclerosis. The prescription cannabis extract nabiximols (Sativex), which contains approximately equal amounts of THC and CBD, is approved in more than 25 countries to treat spasticity and other conditions but is not yet FDA-approved in the US. 

Moderate research support.

Fair-quality research suggests cannabis could be effective for the conditions in this category. However, additional studies are needed to confirm its effectiveness and to elucidate the side effects in people with these specific health conditions.

Anorexia and weight loss. The FDA approved the THC medication dronabinol for stimulating appetite in patients with AIDS in 1992, and recent reviews have found support for this use, although the research is still somewhat limited. The research is insufficient to determine whether cannabis is effective for the treatment of other forms of anorexia such as cancer-associated anorexia or anorexia nervosa.

Sleep disturbances. A 2021 meta-analysis by the Sleep Research Society reported that about 8% of patients report improved sleep quality when taking medical cannabis (orally) or cannabis-based medications, compared to when taking a placebo. The research is based primarily on those with cancer or chronic pain, so it is not known whether a similar pattern exists in the general population. The results are also based on short-term trials, so the long-term effects of using cannabis as a sleep aid are still unknown. Cannabis medications that contain some CBD, like nabiximols, appear to be most effective.

Limited or weak research support.

For the following conditions, there is interest in the treatment potential of cannabis based on findings from animal studies, observational studies, or case reports. However, this research is either conflicting or limited in several important ways, which means that the findings are inconclusive. A substantial amount of additional research is needed to be confident in the effectiveness of cannabis for these conditions.

Opioid addiction: In individuals in treatment for opioid addiction, the majority of studies report that marijuana use does not affect treatment outcomes such as adherence to the program or withdrawal symptoms. In other words, it is neither beneficial nor harmful. Taking prescription cannabinoid medications may be more effective than using marijuana, and these are still under investigation as a remedy for opiate craving. However, these medications would be used in addition to addiction treatment drugs such as methadone and naltrexone, not as a substitute for them.

Anxiety. Anxiety relief is one of the most commonly reported motives for using both medical and recreational marijuana.  A 2020 research update concluded that “the survey data [on anxiety] is generally positive,” but clinical trials using cannabis as a treatment are lacking. Whether cannabis “works” for anxiety depends on whether one is referring to an anxious mood, or a clinical anxiety disorder. Some strains of cannabis have positive effects on mood, but these effects do not extend over the long term. For example, an app called Strainprint collects thousands of self-reports on users’ mood states and the type of cannabis they are using. In the hours after cannabis use (generally by smoking) users report significant decreases in anxiety and stress, but over time (i.e. over the course of months), their baseline levels of stress and anxiety do not improve. This suggests that cannabis may be better suited to replacing short-acting anxiety medications (like beta-blockers), used for performance situations and acute stress, rather than long-term interventions aimed at improving well-being. CBD in particular is being investigated for its short-term effects because it is not intoxicating and thus will not impair cognition or other aspects of performance.

Post-traumatic stress disorder (PTSD). As of 2022 there is only one small placebo-controlled trial on cannabis for PTSD. This 2015 study of ten military personnel found that nabilone, a THC medication, decreased trauma-related nightmares compared to when the same individuals were taking a placebo. Other evidence for a therapeutic benefit of cannabis comes from studies that lack control groups, such as case studies and reviews of medical records. However, the current research is inconclusive. Recent studies support the possibility that cannabis actually worsens PTSD, (or at least certain PTSD symptoms) particularly intrusive memories. Veterans and other trauma survivors are also at very high risk of developing cannabis use disorder. 

Tourette syndrome. Tourette syndrome patients have reported positive effects of cannabis in surveys and case studies, and these reports have sparked interest in drug development. There are currently several cannabis-based medications under investigation for Tourette’s. One is a combination of THC and a compound called PEA; this drug reduced tics in a pilot study of 16 adults, and is now undergoing further trials. 

Another trial is investigating nabiximols, which is a cannabis extract composed of equal parts THC and CBD (already approved for multiple sclerosis and chronic pain). This would be the largest clinical trial to date, with over 90 participants, but was delayed due to the Covid-19 pandemic. A third cannabis-based compound was found to be ineffective in one trial

Both the American Academy of Neurology and the European Society for the Study of Tourette Syndrome consider cannabis medications to be “experimental treatment,” for use only when conventional treatments have failed. This experimental status will likely remain until the publication of results from larger trials.

Traumatic brain injury: Studies in animals have suggested that cannabis might be protective against consequences of traumatic brain injury, such as inflammation and blood vessel damage. However, these findings have not translated into successful human trials. The only large randomized controlled trial to date was conducted in 2006 on a cannabis-based medication called dexanabinol. While dexanabinol was safe, it was not effective in reducing impairments from brain injury at six-month follow-up. Observational studies have found an association between cannabis use (and drug use generally) and decreased risk of mortality following head trauma. However, this may be due to drug users being more likely to be involved in minor accidents than the general population.

Insufficient research support.

For the following conditions there is either an absence of evidence or insufficient evidence to determine whether cannabis or cannabis-based medications have a treatment effect.

Alzheimer’s and other dementia. Several U.S. states have approved cannabis to treat agitation symptoms in people who suffer from Alzheimer’s. However, there is little evidence supporting its effectiveness for this use. There have been only five high-quality trials on THC or THC-medications as a treatment for Alzheimer’s and other forms of dementia. The majority of these reported no effect on cognitive or psychiatric symptoms. A 2021 Cochrane Review concluded that the effects of cannabinoids on dementia are uncertain and any beneficial effects may be too small to be clinically meaningful.

Amyotrophic lateral sclerosis (ALS). In the few trials that have been conducted thus far, THC-based medications do not appear to be effective for the primary symptoms of ALS, though they may be marginally helpful for sleep and appetite. Researchers have turned their attention to medications that incorporate CBD in hopes that these will be more effective. One preliminary trial found that nabiximols (equal THC and CBD) reduced spasticity in ALS, and a larger trial is underway. Another upcoming trial will evaluate a cannabis extract with high CBD and low THC content.

Cancer. There is evidence from animal studies that cannabinoids can selectively kill cancerous tumor cells and may augment the effects of chemotherapy drugs. Several case reports also suggest a positive effect of CBD. However, since the research on humans is very limited, the scientific consensus is that there is insufficient research to determine whether cannabis has an effect on preventing or treating cancer. Clinical trials on cannabis-based medications in humans are just beginning, with a preliminary trial published in 2021 that will likely pave the way for larger trials.

Depression. There are currently no registered clinical trials on cannabis or cannabis-based medications in the treatment of Major Depressive Disorder, so there is no quality evidence regarding the treatment of moderate or severe depression. Studies on depressive symptoms in people using cannabis to treat other disorders have produced mixed results. In the general population, there are widespread reports of individuals using cannabis to self-medicate depressive symptoms. One study using data from the app Strainprint found that users reported short-term decreases in depression immediately after using cannabis but slight worsening of depression with long term use.

Dystonia (a neurological movement disorder). There is insufficient evidence to determine whether cannabis is effective in the treatment of dystonia. There have not been any full scale clinical trials investigating the issue.

Huntington’s disease. There is insufficient evidence to support the use of cannabis for treatment of symptoms from Huntington’s disease. There have not been any clinical trials using adequate numbers of patients to date.

Parkinson’s disease. The NASEM found insufficient evidence to support the use of cannabis to help with the motor symptoms in patients who suffer from Parkinson’s. There have not been any clinical trials conducted with adequate numbers of participants. Two preliminary trials did not find a benefit of cannabinoids for Parkinson’s symptoms.

Irritable bowel syndrome. There is a lack of research supporting the use of cannabis as a treatment for irritable bowel syndrome. Very few studies have been conducted on the subject and the majority report that cannabis and cannabis extracts are not effective in inducing remission in IBS.

Cannabis Is Not Effective for Some Conditions

Glaucoma. The American Glaucoma Society and the American Academy of Ophthalmology both maintain the position that there is currently “no scientific basis for use of [cannabinoids] in the treatment of glaucoma.” The treatment utility of cannabis is limited by its short duration of action; glaucoma requires long-acting medications in order to stabilize symptoms. While THC has been investigated for its ability to lower pressure in the eye, other components of marijuana (including CBD) may be harmful to ocular health.

Side effects of medical cannabis

Research has looked more into the potential adverse effects of cannabis than it has the potential therapeutic effects. The upside of this is that it is now possible for individuals to make a more informed decision about risks given their personal and family medical history. 

Vaping. and e-cigarettes are sometimes used as a delivery for weed and cannabis derivatives. A 2020 paper cautions that “while some vaporizers and vape pens can be used with relatively unprocessed cannabis…many vaping devices use highly processed products whose safety and chemical profile are much closer to that of e-liquids used in e-cigarettes.” There are significant side effect risks associated with vaping liquid concentrates, especially those that contain vitamin e acetate. Of these, lung injury is one of the most serious concerns.

Pregnancy and fetal exposure. Cannabis use is reliably associated with low birth weight independent of other substance use, and greater reductions in birth weight are seen with more frequent use. And it’s not just the mothers. There’s new evidence (though more research is needed) that when fathers use cannabis prior to conception, it may cause changes in gene expression on the sperm that raise the risk of problems such as low birth weight, spontaneous abortion, and neurodevelopment changes.

Preterm delivery is also associated with cannabis use but this effect is believed to be driven by tobacco.

THC can cross the placental barrier and affect brain development in the fetus. Longitudinal studies on children born in the 1970’s, 80’s, and 90’s suggest that prenatal cannabis exposure is associated with impulsivity and impairments in certain cognitive functions later in life. Some researchers argue that these impairments are mild and limited in scope. However, because of dramatic increases in the THC content of marijuana in recent decades, there is concern that modern strains have the potential to be more toxic to the developing brain than strains used in the past. Newer studies are needed to confirm whether this is the case. In addition, the impact of CBD on brain development is not yet known.

Bipolar disorder. Research on cannabis and bipolar disorder is in its early stages, but results so far consistently show that cannabis use increases the risk of having a manic or hypomanic episode. This is not the case for other common recreational substances; for example, alcohol consumption is not associated with mania. Cannabis can play a role in both the development and maintenance of bipolar disorder; that is, it can play a role in triggering someone’s first manic episode and also increase the likelihood of subsequent episodes. The risk of a manic episode increases with greater frequency of cannabis use and with the use of high-THC strains. 

Psychotic disorders. Frequent marijuana use is a significant risk factor for the development of schizophrenia and other psychotic disorders. Daily cannabis users are two to four times more likely to develop psychosis than non-users, and the risk is even higher if the individuals use high-THC strains. Cannabis use interacts with other risk factors such as genetic liability and environmental stressors; some people are at such high genetic risk that they will likely develop schizophrenia regardless, whereas for others, marijuana plays the most significant role in triggering their disorder. (However even in high-risk individuals, marijuana can cause their disorder to manifest earlier in life).  Rare use (monthly or less) is believed to be safe for most people in regards to these outcomes, and moderate use (weekly) represents an intermediate level of risk.

Cancer. Current research suggests that using cannabis does not increase a person’s risk for cancers of the head and neck, and does not increase the risk of lung cancer independent of tobacco use. Research on other types of cancer, such as bladder, prostate, and cervical cancer is inconclusive because of the limited number of studies available. In contrast, there is evidence that men who used marijuana prior to completing puberty are at increased risk of testicular cancer. This may be because THC alters the activity of brain regions responsible for regulating hormones and sexual development.

Cannabis use disorder. Cannabis use disorder is diagnosed when a person’s use impacts their ability to function in their work, school, and relationships. Disordered use also frequently involves dangerous behaviors such as driving under the influence. Contrary to the popular belief that cannabis addiction is rare, surveys have shown that 10% to 30% of adult marijuana users develop this disorder. Factors such as frequency of use, potency of the drug, and genetic, social, and economic conditions impact a person’s risk. The “addictive potential” of cannabis is similar to that of alcohol, but much lower than that of nicotine. However, features like withdrawal symptoms and multiple failed attempts to quit are characteristic of cannabis use disorder as well as other substance addictions.

Cardiovascular risk. Cannabis use may result in increased risk of cardiovascular events like heart attack and stroke. Because these events are rare and sometimes fatal, there are ethical and logistical barriers to conducting experiments on them, so available evidence is based on correlations. However, analyses of hospital records do show an association between cannabis use and incidence of heart attack and stroke. 

Another 2018 analysis found an increase in the rate of cardiovascular mortality in states that had recently legalized medical marijuana. These risks may be dependent on dose and frequency of use; for example, in one cohort an increased risk of stroke was observed in people who smoked marijuana once per week or more, but not in less frequent users. For stroke in particular this association has not only been observed in older people with existing risk factors but also in young people with no known risk factors. The American Heart Association advises that though smoking marijuana may be worse for long term health, “edible consumption appears more likely to result in [acute] adverse effects, particularly psychiatric and cardiovascular.”

Respiratory disease. Regular cannabis smoking is associated with symptoms of chronic bronchitis, such as increased phlegm production, wheezing, coughing, and shortness of breath. Smoking marijuana is also linked to increased susceptibility to lung infections, including tuberculosis and Covid-19. Unlike tobacco, marijuana alone has not been shown to be a risk factor for lung conditions such as emphysema. However, there is evidence that the combination of marijuana and tobacco use raises the risk of these conditions beyond that posed by tobacco alone.. Additionally, a recent survey found that respondents who reduced their marijuana use were more likely to successfully quit tobacco.

Cognitive impairment. There is very good evidence that the use of cannabis can lead to short-term cognitive impairments in learning, memory, attention, reasoning, and judgment. These impairments can be long-lasting for both adolescents and adults who use marijuana regularly.

Injury and motor coordination. Use of marijuana is associated with impaired motor coordination and accidents, including motor vehicle accidents. Cannabis consumption roughly doubles the risk of being involved in a fatal car crash.

Mood disorders. While mood disorders and substance use often co-occur, there is no strong evidence that cannabis directly influences the risk of developing depression or anxiety. Researchers believe it is more likely that depressed and/or anxious individuals use cannabis to self-medicate, but this is an ineffective strategy that does not produce long term remission. Individuals who use cannabis in the context of anxiety or depression may lack access to effective mental health treatment. 

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 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 37 states and Washington D.C. Each state has different rules around how patients can be prescribed medical cannabis and how it can be obtained. Recreational marijuana is now legal in 19 states, and The National Council on State Legislatures’ Cannabis Overview is frequently updated with the latest information on state legalization. In deciding whether to use cannabis for any reason, it is important to consider the legal and employment risks of using it in your state.

Even though medical cannabis is lawful in many states, it is still illegal at the federal level. Cannabis is listed as a Schedule 1 substance in the Controlled Substances Act. 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. 

However the Rohrabacher–Farr amendment, which was renewed in 2022, prevents the federal government from interfering in state laws governing medical marijuana use. In practice, doctors in these states are protected from legal action for prescribing cannabis or cannabis-based medications in accordance with their own state laws. If your state permits medical marijuana, your doctor should be able to discuss whether cannabis is an appropriate treatment option for you.

This article, originally published in 2020, was updated in 2022 by Joyce Clanon.