Support for medical cannabis has skyrocketed in recent years, with suggestions that it can help with certain conditions, but despite its long history – it’s been used as a remedy from as far back as 2700BC – still little is known about its health effects.
Back in 2700 BC, Emperor Shen Nung – the father of Chinese medicine – claimed cannabis could treat ailments from absent-mindedness to malaria, and before being outlawed in 1942 in the US, cannabis was listed in the national pharmacopoeia, a compendium of commonly used medicines.
But, reports New Scientist, we don’t know much about its health benefits, if any, mainly due to the strict regulations around research, which are only starting to lift.
There are some promising signs in the findings, however, and new insights are emerging regularly. So how can we know when marijuana is good medicine?
Why cannabis has medical potential
A growing pile of research shows 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.
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.
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, 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.
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 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 by 7% to 30% 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% of those treated with a THC-and-CBD mouth spray survived after a year versus 44% of those given a placebo.
The anti-cancer effect is much more convincing at the animal level, as there haven’t been a lot of clinical trials in people. There are also some interactions between CBD and certain chemotherapy drugs, so those undergoing cancer treatment should inform their doctor of marijuana use.
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% of people given cannabinoids saw their pain decrease by about a third versus 26% of those given a placebo. About 18% of the cannabinoid group and 14% of the control group saw their pain cut in half. A note of caution: heavy cannabis use may worsen pain. A survey of almost 1 000 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. In other words, too little or too much marijuana may heighten pain.
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 8 500 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% on average, whereas those given a placebo saw a 15% 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%. 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.
Insomnia
Small trials suggest medical marijuana may improve sleep. For example, a 2022 study of 29 people with self-reported insomnia found that 60% 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% more likely than those who didn’t to sleep less than six hours a night. They were also 56% more likely to sleep longer than nine 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 2 000 people found that, on average, medical marijuana more than halved the number of migraine headaches that had each month. Additionally, almost 12% 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 causing 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 1 000 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.
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.
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.
Yet for many of these ailments, cannabis research is just beginning, and there are many other evidence-backed treatment options, say experts.
New Scientist article – Marijuana as medicine: Everything you need to know (Open access)
See more from MedicalBrief archives:
Foetal growth stunted by cannabis in pregnancy – US study
Cannabis for cancer-related symptoms – an unfulfilled promise?
Little evidence of cannabis efficacy for chronic pain relief – US DoH systematic review
Dosing, administering medical cannabis for chronic pain – Recommendations