Marijuana as medicine: Everything you need to know

Medical marijuana use is skyrocketing: between 2016 and 2020, the number of people in the US with a licence to buy it more than quadrupled, reaching nearly 3 million – a figure that excludes people self-medicating outside official programmes. There is a similar upward trend in Australia. In the UK, where medical cannabis was legalised in 2018, just 60 prescriptions had been issued by 2020, but an estimated 1.4 million people say they used it for medicinal purposes that year.

Humans have long used cannabis as a remedy, of course. As far back as 2700 BC, Emperor Shen Nung – the father of Chinese medicine – claimed cannabis could treat ailments from absent-mindedness to malaria. In Western medicine, cannabis became popular in the late 19th and early 20th centuries. One of Queen Victoria’s doctors even used it to treat her premenstrual symptoms. Before it was outlawed in 1942 in the US, cannabis was listed in the national pharmacopoeia, a compendium of commonly used medicines.

Despite this long history, we still know relatively little about its health effects. That is mainly due to the strict regulations around research, which are only starting to lift.

There are some promising signs in the research however, and new insights are emerging regularly. So how can we know when marijuana is good medicine?

In this guide, we focus on 20 conditions that public health authorities identify as being under active investigation or showing potential benefits from treatment with medical cannabis. We sifted through the latest research to determine where there is strong evidence of benefit, where the picture is more mixed and where cannabis could make matters worse.

Why cannabis has medical potential

There is a growing pile of research showing that cannabis compounds can increase appetite, promote feelings of relaxation and even induce euphoria. These and many other effects are mainly mediated by a cell-signalling network in the body known as the endocannabinoid system. Chemical compounds known as cannabinoids are produced not only by cannabis plants, but also by our own bodies, and both versions act on this complex system.

The endocannabinoid system was discovered in the 1980s and is known to help regulate essential processes like hunger, sleep, mood and body temperature. Recently, dysfunction in this system has also been implicated in conditions including Parkinson’s disease and multiple sclerosis, suggesting potential new avenues for treatment. Scientists have discovered more than 100 cannabinoids in marijuana, many of which we are just beginning to understand. The most well-known are tetrahydrocannabinol, or THC, and cannabidiol, or CBD. Experiments in animals show that both chemicals have anti-inflammatory and antioxidant properties, which may be behind their therapeutic potential.

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As the use of marijuana and its compounds rises around the world, New Scientist explores the latest research on the medical potential of cannabis, how it is grown and its environmental impact, the way cannabis affects our bodies and minds and what the marijuana of the future will look like.

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But cannabinoids are only the tip of the iceberg, says Kevin Boehnke at the University of Michigan. Marijuana also contains molecules known as flavonoids and terpenes, he says, which are found in fruits and vegetables and are known to have health benefits, too.

“If we discovered [cannabis] today and it didn’t have all the baggage we now associate with it, we’d be like, ‘Holy smokes. This is an incredible plant’,” says Boehnke. “But we need to go into it with open eyes. This is not a panacea. It’s not going to provide symptom relief for everyone or for every condition.”

Jump to condition

Alzheimer’s disease
Amyotrophic lateral sclerosis (ALS)
Anxiety
Attention-deficit/hyperactivity disorder (ADHD)
Cancer
Chronic pain
Depression
Epilepsy
Glaucoma
HIV symptoms
Inflammatory bowel disease (IBD)
Insomnia
Migraine
Multiple Sclerosis (MS) symptoms
Nausea
Opioid use disorder
Post-traumatic stress disorder (PTSD)
Schizophrenia
Tourette syndrome (TS)
Tuberous sclerosis complex (TSC)

Alzheimer’s disease

The underlying cause of Alzheimer’s disease is still unknown, but one hallmark of its progression is the accumulation of abnormal proteins, called amyloid plaques, in the brain.

One study found that THC may prevent amyloid plaques from building up in human brain cells in a dish. Another study, which screened 11 cannabinoids for neuroprotective properties, showed that nine of them, including THC and cannabinol (CBN), could protect animal and human brain cells from neurological toxins like amyloid plaques. Experiments in rats with signs of Alzheimer’s also found that CBD and low-dose THC improved cognition in the ageing animals.

These findings suggest that some cannabinoids might hold promise in preventing or slowing Alzheimer’s progression. However, this hasn’t yet been shown in people with the condition.

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Amyotrophic lateral sclerosis (ALS)

Nerve cells in the brain and spinal cord deteriorate in ALS, causing muscle weakness, stiffness and cramping along with loss of movement. In mice with signs of the disease, cannabis prolongs the survival of nerve cells and slows progression of symptoms. A 2019 study involving 59 people with ALS showed that people who took an oral spray containing THC and CBD saw slightly improved muscle stiffness after six weeks, whereas those taking a placebo saw symptoms worsen. However, larger trials are needed before cannabis can be recommended as an ALS treatment.

Anxiety

Getting high often induces feelings of relaxation thanks to the stimulation of endocannabinoid receptors, which may reduce anxiety. This could explain why observational studies show that people with anxiety who use cannabis report greater improvements in symptoms than those who do not.

Yet robust evidence is lacking. For instance, a 2022 trial of 32 college students with moderate-to-severe anxiety related to taking tests found that those who were given CBD during a mock statistics exam had similar anxiety levels to those who weren’t. Another 2022 trial involving 186 adults with pain, insomnia, anxiety or depressive symptoms found no difference in anxiety between those with a medical marijuana card and those without one. Evidence also suggests cannabis, especially that containing high amounts of THC, can worsen anxiety.

Attention-deficit/hyperactivity disorder (ADHD)

There have been a few case reports of people with ADHD successfully using cannabis to treat their symptoms. For instance, in three adults with the condition, measures of inattention improved by 7 to 30 per cent after they integrated the drug into their treatment regimen. Another adult with ADHD saw improvements in concentration, boredom and angry outbursts after being prescribed two medications with THC, CBD or both. However, most studies show cannabis has no effect on ADHD and could actually worsen symptoms, since it impairs attention and memory.

Cancer

Preliminary evidence suggests some cannabinoids may have anti-tumour properties, especially when paired with certain chemotherapy drugs. For instance, a study in mice with pancreatic cancer found that those treated with CBD and the chemotherapy medication gemcitabine survived three times as long as animals treated with gemcitabine alone. Meanwhile, a trial involving 21 people with an aggressive form of brain cancer found that 83 per cent of those treated with a THC-and-CBD mouth spray survived after a year versus 44 per cent of those given a placebo.

“The anti-cancer effect is much more convincing at the animal level. They haven’t don’t a lot of clinical trials [in people],” says Holland. There are some interactions between CBD and certain chemotherapy drugs, so those undergoing cancer treatment should inform their doctor of marijuana use, she says.

There is also a mouth spray with THC and CBD available in Canada and parts of Europe for treating cancer-related pain.

Chronic pain

Inflammation underlies many chronic pain conditions, including fibromyalgia and arthritis. Several studies have now shown that cannabinoids, including CBD, THC and cannabigerol (CBG), can reduce inflammation. Cannabis may, therefore, benefit people with inflammation-related pain – though, so far, evidence suggests that there is only a small effect.

For instance, a 2018 review of 47 randomised controlled trials showed that 29 per cent of people given cannabinoids saw their pain decrease by about a third versus 26 per cent of those given a placebo. About 18 per cent of the cannabinoid group and 14 per cent of the control group saw their pain cut in half. “[With cannabis] you need lower doses of opiates to get pain control. But on its own, as a painkiller, the evidence is not strong,” says Holland.

A note of caution: heavy cannabis use may worsen pain. A survey of almost 1000 adults with chronic pain who used cannabis daily found that more frequent use was associated with worse pain. This is known as a biphasic effect, says Boehnke. In other words, too little or too much marijuana may heighten pain, he says.

Depression

Since the endocannabinoid system plays an important role in regulating mood, some have proposed cannabis as a potential treatment for depression. However, evidence is mixed and a handful of studies indicate that cannabis could worsen depressive symptoms.

A 2022 trial had 105 adults with pain, insomnia, anxiety or depressive symptoms obtain a medical marijuana card while a separate group of 81 people waited 12 weeks before doing so. There was no significant difference in depressive symptoms between the two groups during this period. Another study tracking depressive symptoms in more than 8500 people in Switzerland for three years found no difference in rates of depression between those who used cannabis and those who didn’t.

Epilepsy

In the US, UK, European Union and Australia, Epidiolex, a medication containing CBD, is approved to treat two rare forms of epilepsy – Lennox-Gastaut syndrome and Dravet syndrome. In clinical trials, the drug decreased the frequency of seizures in people with either condition by about 40 per cent on average, whereas those given a placebo saw a 15 per cent reduction. Research in mice showed that CBD prevents seizures by blocking a molecule known to amplify nerve signals in the brain. Yet so far, there is minimal evidence that cannabis compounds are useful to treat other, more common forms of epilepsy.

Glaucoma

Glaucoma occurs when fluid builds up in the eyes, resulting in pressure that damages a nerve crucial for vision. Small trials have shown that in most people with glaucoma, marijuana can alleviate eye pressure by about 25 per cent. This is potentially due to cannabinoids binding with CB1 receptors in the eye. However, the effect only lasts a few hours and isn’t as robust as standard treatments.  As such, the American Academy of Ophthalmology recommends against cannabis for glaucoma.

HIV symptoms

There is some evidence that cannabis may alleviate symptoms of HIV infection, which can linger or persist after people begin antiviral treatment. For instance, the FDA approved a THC medication called Dronabinol for HIV-related weight loss after small trials showed it increased hunger hormones in people with the virus.

Researchers are also investigating whether the anti-inflammatory effects of some cannabinoids could help with brain inflammation in those with HIV, which leads to memory and attention problems. A study of nearly 200 people with HIV found that those who regularly used cannabis had lower levels of inflammatory molecules in their spinal fluid compared with those who didn’t. A separate study also showed that cannabis use in people with HIV was associated with fewer cognitive impairments. These papers were merely observational, though, and scientists will need well-designed trials with a control group to confirm any benefits.

Inflammatory bowel disease (IBD)

IBD is a chronic, inflammatory condition affecting the gut. It causes abdominal pain, diarrhoea and weight loss. Because cannabis has anti-inflammatory effects, it has been proposed as a potential treatment for IBD. A handful of small trials have shown cannabis can reduce symptoms of IBD, primarily abdominal pain. Even so, remission rates between those given the drug and those given a placebo aren’t statistically different. There is also limited evidence that it actually reduces inflammation caused by the condition.

Insomnia

Small trials suggest medical marijuana may improve sleep. For example, a 2022 study of 29 people with self-reported insomnia found that 60 per cent of them no longer met insomnia criteria after two weeks of using a THC-and-CBD oil. When using the preparation, participants had, on average, 21 additional minutes of light sleep per night than when using a placebo. The study suggests the benefit may be due to cannabinoids increasing levels of the hormone melatonin, though the relationship between melatonin and a good night’s rest is more complicated than it seems.

Another trial of 186 adults with signs of insomnia, pain, anxiety or depression found that those with a medical marijuana card, which allows the legal purchase of marijuana for medical purposes, reported fewer insomnia symptoms after 12 weeks than those without one.

Yet, cannabis use has also been associated with worse sleep. A survey of more than 21,000 adults in the US who used marijuana showed that they were 34 per cent more likely than those who didn’t to sleep less than 6 hours a night. They were also 56 per cent more likely to sleep longer than 9 hours a night.

Migraine

Some research suggests the endocannabinoid system plays a role in mitigating migraines, potentially by regulating inflammation and reducing pain.

A 2022 review of 12 studies involving almost 2000 people found that, on average, medical marijuana more than halved the number of migraine headaches that had each month. Additionally, almost 12 per cent of participants no longer experienced migraine headaches when using cannabis. After six months, the drug also reduced nausea and vomiting associated with migraines. On the flip side, there is some evidence that overusing marijuana may induce headaches.

Multiple Sclerosis (MS) symptoms

Multiple sclerosis is a chronic neurodegenerative condition that causes difficulties with walking, memory and other bodily functions. A THC-and-CBD mouth spray called Sativex has been approved for multiple sclerosis in more than 25 countries, including the UK and Canada.

Three clinical trials involving more than 1000 people with MS assessed how the drug affects muscle stiffness on a 10-point scale. Average scores between those given the spray and those given a placebo differed by less than 1 point. However, five studies totalling more than 500 people with MS found that, on average, the cannabinoid spray reduced pain scores by more than 3 points on an 11-point scale. Research indicates that cannabis may improve bladder issues and sleep problems related to MS as well.

While multiple trials show that THC and CBD improve MS symptoms, the effect is small. There is also no convincing evidence that cannabinoids can slow or reverse the disease’s progression.

Nausea

Multiple studies show that cannabis can reduce nausea and vomiting. In fact, the FDA has approved Dronabinol, which contains THC, for treating nausea and vomiting in people with cancer undergoing chemotherapy. One study had 64 people receiving chemotherapy take either Dronabinol, ondansetron – an anti-nausea medication – or a placebo after treatment. It found that 71 per cent of those given Dronabinol, 64 per cent given ondansetron and 15 per cent given a placebo didn’t experience chemo-induced nausea.

In a 2022 study, 886 people seeking treatment for nausea recorded the degree of nausea symptoms after taking cannabis between June and July 2016. One hour after taking cannabis, almost 95 per cent of people experienced nausea relief, with an average reduction in symptom severity of almost 4 points on a 10-point scale.

However, daily, long-term cannabis users can develop cannabinoid hyperemesis syndrome, a condition that leads to repeated and severe bouts of vomiting. There may also be a risk of using marijuana for nausea during pregnancy, as cannabinoids could affect fetal development.

Opioid use disorder

Given the worsening opioid epidemic in the US, one of the most promising effects of medical marijuana is preventing or treating opioid misuse. While the impact of cannabis legalisation on the ongoing opioid overdose epidemic has been debated, there is a growing body of research supporting its potential to help treat people with opioid use disorder.

A 2020 study estimated that if US counties with a marijuana dispensary opened another storefront, they could decrease opioid-related deaths by 17 per cent. This may be for two reasons. First, there is some evidence that CBD can reduce drug cravings in people addicted to heroin. Second, prescribing medical marijuana alongside opioids for chronic pain can cut opioid use. A 2023 study of more than 8000 people prescribed opioids showed that those using cannabis approximately halved their opioid use after eight months. Meanwhile, those who only used opioids saw less than a 15 per cent reduction.

“Cannabis is sort of like the condom of drug policy,” says Julie Holland, a psychiatrist with her own practice in New York City. “It’s a very clear harm reduction method that decreases the amount of people that die [from opioids].”

Post-traumatic stress disorder (PTSD)

PTSD is a mental condition that develops after someone experiences a traumatic event. A few cannabinoid receptors are located in the same brain regions involved in PTSD, so some researchers think medical cannabis may improve symptoms of the condition by stimulating those receptors.

A 2021 study followed 150 people with PTSD for a year and found that those who used cannabis were about 2.5 times more likely to no longer meet criteria for the condition. Yet a trial of 80 people with PTSD showed that, compared with a placebo, cannabis had no significant effect on symptoms after three weeks.

Ultimately, the available evidence isn’t sufficient to support cannabis as a PTSD treatment, especially since those with the condition are more likely than those without it to experience cannabis withdrawal symptoms and cravings.

Schizophrenia

People with psychotic disorders, including schizophrenia, and those who have an increased risk for such conditions shouldn’t use cannabis. A 2018 review of 66 research papers found that cannabis doubles the risk of developing psychosis in vulnerable people. Some evidence indicates this could be due to cannabis altering levels of inflammatory molecules in people with schizophrenia. Other research has shown a link between impaired functioning of certain cannabinoid receptors and the condition.

Tourette’s syndrome (TS)

Tourette’s syndrome is characterised by uncontrollable and repetitive movements or sounds called tics. It affects the nervous system, especially a region of the brain rich in cannabinoid receptors called the basal ganglia. A 2023 study investigated the effects of THC in 22 people with the condition. It measured the severity of tics using a 50-point scale. After six weeks, tic severity improved by an average of almost 9 points in the THC group and less than 3 points in the placebo group. While these results are promising, other clinical trials have shown no effect.

Tuberous sclerosis complex (TSC)

In this rare genetic disease, non-cancerous tumours grow in the brain and other areas of the body, often leading to seizures. The US Food and Drug Administration (FDA) approved the CBD-containing drug Epidiolex for the condition after a clinical trial showed it reduced the frequency of seizures in people with TSC by almost 50 per cent, on average.

Taken together, the evidence is clear that cannabis and its many compounds show potential in alleviating or even treating a wide range of health conditions. “The safety, the diversity of the plant and the myriad functions that the endocannabinoid system is involved with makes [marijuana] a great potential target,” says Boenke. Yet for many of these ailments, cannabis research is just beginning, and there are many other evidence-backed treatment options. If you are considering trying medical cannabis, it is also important to consult a doctor because cannabis can interfere with other medications and may not be suitable for everyone.