Cannabis has a rich history both as a plant (with estimates dating it back as far as 28 million years) and with regard to human cannabis cultivation.  Humans have been cultivating cannabis plants for food, medicine, fiber, fuel, and religious purposes for more than 10,000 years in Europe and Asia (11,12).  

Cannabis plants are believed to have originated in Central Asia, in regions such as Mongolia and southern Siberia. Buried cannabis seeds have been discovered in kurgan burial mounds in Siberia dating as far back as 3,000 B.C.  Cannabis has an extensive history for medicinal and spiritual purposes.  The ancient Greeks used cannabis to dress wounds on their horses, and cannabis seeds were also used for expelling tapeworms (13).  In ancient India, cannabis was a large part of religious practices and was also seen as a powerful medicine. Ancient Indian texts confirm cannabis was used for treating conditions such as insomnia, headaches, and pain from childbirth (14).  In ancient China hemp fiber was used to make paper, clothing, and bowstrings.  It was used as a medicine to treat a variety of conditions, as well as a surgical anesthetic.  Evidence suggests the psychoactive effects of cannabis smoke may have been a part of traditional Chinese burial rituals (15).   Ancient Egyptians also used cannabis for many medical and religious purposes. In the Himalayas of Tibet, cannabis facilitated deep meditation in Tantric Buddhism rituals (16).

Today over half of the U.S states and other countries such as Australia, New Zealand, and the U.K. have legalized medical cannabis due to the widespread scientific consensus for its medicinal value.

Research shows that cannabis helps decrease substance abuse, epileptic seizures, pain, and can alleviate some side effects of cancer chemotherapy treatment. Research also suggests that cannabinoids may help slow the progression of Alzheimer’s disease, help reduce symptoms of Post Traumatic Stress Disorder (PTSD) and adult attention deficit hyperactivity disorder (ADHD).  In addition, states in the US that enacted medical marijuana laws reported decreases in prescriptions to treat the following conditions: pain, seizures, depression, psychosis, anxiety, nausea, sleep disorder, and spasticity (17).  This observation may provide additional insight into cannabis’ therapeutic potential.

Cannabis appears to help ease the symptoms of opioid withdrawal. Population studies also show opioid-related deaths have decreased in U.S states that have legalized cannabis. CBD may also help decrease tobacco use in smokers.

• In one review on the role of cannabis in opioid use disorder, researchers concluded that cannabis may help to decrease symptoms of opioid withdrawal (18).
  
  
• Cannabis appears to be associated with a decrease in opioid-related deaths and overdoses in U.S states such as Colorado which have legalized cannabis (19).
  
  
• In one study, tobacco smokers who used a CBD inhaler smoked 40% fewer cigarettes, while there was no observable difference in the placebo group (20).

Dravet syndrome is a type of epilepsy that is marked by prolonged seizures that usually begin before 1 year of age and  is very difficult to manage with anti-convulsant medication. Lennox-Gastaut syndrome is a  rare type of childhood epilepsy, with an onset of 3-5 years.  Children with Dravet syndrome, and LGS experienced nearly 40% decreases in seizure frequency following CBD administration.

• A 2017 study tested CBD as a treatment for patients with Dravet syndrome (21). The results showed that out of 120 children aged between two and 18, those who were given a 20 mg per kg of body weight dose of CBD in addition to their normal drug routine experienced a 39% reduction in monthly seizure frequency, compared to only 13% of the placebo group.
  
  
• In a double-blind randomized placebo-controlled trial published in 2018, 225 patients with Lennox-Gastaut syndrome experienced a 37% reduction in seizures when given a 10 mg dose of CBD (22). The placebo group experienced 17% reductions.

The healing properties of CBD may have application in preventing the progression of Alzheimer’s disease. THC appears to relieve symptoms of dementia, such as aggression and agitation

• Several studies suggest that CBD may help prevent Alzheimer’s disease progression due to its neuroprotective, anti-inflammatory and antioxidant properties (23,24).
  
  
• In one study, CBD was concluded to prevent impaired social recognition in adult mice with Alzheimer’s symptoms (25).
  
  
• In a 2014 cohort study on 40 patients with dementia, a high dose of the THC analog Dronabinol decreased symptoms of aggression and agitation (26).
  
  
• Two years after the 2014 cohort study, an Israeli team reported similar results in a group of 11 patients with Alzheimer’s who were administered cannabis oil that contained THC and varying quantities of CBD (27).

There is preliminary evidence that cannabis use may contribute to decreasing severe depressive and suicidal states associated with PTSD.  In addition, military veterans reported cannabis helped reduce anxiety and insomnia associated with PTSD.  

• According to one 2015 meta-review, a large number of military veterans with PTSD used cannabis successfully to treat their PTSD symptoms (28). These veterans say that cannabis reduced their anxiety, helped with insomnia, nightmares and made coping with emotional trauma easier.
  
  
• In the 2012 Canadian Community Health Survey, 420 (1.7%) respondents reported a current clinical diagnosis of PTSD, and 106 (28.2%) of these PTSD respondents reported cannabis used within the past year. While PTSD was significantly associated with recent major depressive episode and suicidal ideation, this association was not associated with cannabis-using respondents with PTSD (both p > 0.05) (29).

It has been well-established that the endocannabinoid system plays an important role in mood regulation.  Research on cannabis and depression has produced mixed results, suggesting that more nuanced studies are needed to understand the factors contributing to the variability of these effects.  There is evidence that suggests heavy cannabis use may be detrimental for people with major depressive disorder (MDD).   

• In one 2016 review of using cannabis for depression, researchers found nine studies on cannabis and depression (30). Seven of the nine studies showed that patients experienced improved symptoms of depression with cannabis use.
  
  
• One meta-analysis looking at the long-term association between cannabis use and depression found a greater level of depressive symptoms in heavy cannabis users compared to light-users and non-users (31).
  
  
• One clinical study showed that reducing cannabis use causes less depressive symptoms (32).
  
  
• A cross-sectional survey on patterns of use and perceived efficacy of cannabis suggested that in over 1429 participants identified as medical cannabis users, over 50% reported using medicinal cannabis specifically for depression (33).

The ECS is a modulator of anxiety and mood. Cannabis produces what is referred to as a ‘biphasic effect’, meaning a lower dose may reduce anxiety, while a higher dose may trigger it (34).  Though typically high levels of THC are thought to provoke anxiety, this effect may be reduced by CBD (35).  Though more research is needed, especially looking at long-term effects,  CBD holds promise as an alternative therapy for anxiety management (36).

• One small preliminary double-blind RCT compared the effects of a simulated public speaking test on patients with social anxiety (n = 24) versus controls (n = 12) (37). Each group received a single acute oral dose of CBD (600 mg) 1.5 h before the test, or matching placebo. Treatment with CBD significantly reduced anxiety, cognitive impairment and discomfort in the social anxiety group’s speech performance, and significantly decreased hyper-alertness compared to controls.  
  
  
• In one study 10 patients with social anxiety were given 400 mg of oral CBD or placebo in a double-blinded crossover manner. The CBD-treatment group was associated with significantly decreased anxiety, and with changes in blood flow in the brain, suggesting CBD’s activity may be due to the interaction with the limbic and paralimbic brain areas (38).

Though cannabinoids have some anti-cancer properties, the role they play in cancer formation and treatment is not clear and requires further research. Cannabis and its synthetic form, dronabinol, help to ease nausea following cancer chemotherapy.

• Despite some claims, there is very little evidence showing that cannabis is a cure for cancer (39).
  
  
• Studies do show that cannabinoids like CBD and THC may have anti-cancer properties, specifically with regards to skin, breast, blood, liver and throat cancers in animal and in vivo (test-tube) studies (40,41,42,43, 44).
  
  
• The synthetic form of cannabis, Dronabinol, is prescribed specifically to help treat severe nausea and vomiting, which are two of the most common symptoms of cancer chemotherapy (45).

Pain is the most common condition people use medical cannabis to treat. Cannabis can treat chronic pain, neuropathic pain, and can improve pain associated with cancer in patients that do not respond to opioid treatments.

• One 2015 systematic review of 28 studies concluded that there was “there was moderate-quality evidence to support the use of cannabinoids for the treatment of chronic pain.” (46).
  
  
• One highly-cited randomized placebo-controlled trial from 2008 examined the effects of smoking cannabis on neuropathic pain. The results showed that smoking cannabis caused significant relief (47).
  
  
• One randomized controlled trial published in 2010 examined the effects of THC-CBD combination treatments in patients with pain from cancer who weren’t responding to regular opioid medications (48). The results showed that 43% of subjects reported a 30% or greater improvement in pain symptoms.

The cannabinoid THC has been shown to increase appetite, while CBD seems to have the opposite effect. These effects have applications in improving appetite dysfunction in cancer patients and those with eating disorders such as anorexia.

• The appetite-increasing effects of cannabis may help cancer patients experiencing a decrease in appetite from chemotherapy eat a sufficient amount of calories, thereby improving their health and quality of life (49).
  
  
• People who have eating disorders such as anorexia may benefit from the increase in appetite THC causes. Increasing a patient’s appetite may help them gain weight and therefore decrease their risk of adverse effects from malnutrition (50).

In adults with ADHD the cannabinoid, cannabinol (CBN), was associated with decreases in self-reported ADHD symptoms, and consumption of medical cannabis was linked to decreases in taking ADHD medication (51).

No deaths have ever been directly attributed to the toxicity of cannabis. In general, cannabis is well-tolerated for adults, especially CBD, which has been stated to be safe to use by the World Health Organization (WHO) (52,53).

Despite cannabis being well-tolerated for most people, there are risks one must consider. Chronic smoking of cannabis is associated with compromised lung and cardiovascular function (54). Chronic use of cannabis may also increase symptoms of depression, trigger underlying psychiatric conditions such as schizophrenia, and cause cognitive deficits in adulthood for those who chronically use cannabis in their younger years or who have a family history of such conditions (55,56). High frequency cannabis use may result in dependency.

In addition women who are pregnant or wish to become pregnant should avoid cannabis, as there is evidence that cannabis may block the growth of new blood vessels, which could prevent egg implantation, lead to miscarriage, or alter fetal development (57, 58).  It is wise for elder users to take proper precautions as they may be more sensitive to the effects of cannabis, and it may affect their balance.

You should not take cannabis if you:

• Are a teenager who doesn’t need it for medical reasons
• Feel pressured to take cannabis because of others
• Are pregnant or wish to become pregnant
• Are breastfeeding
• Are predisposed to developing mental illnesses such as schizophrenia, bipolar disorder or borderline personality disorder
• Have a severe cardiovascular condition
• Have hepatitis C

Pharmacological Contraindications

You can read more about potential adverse drug interactions with cannabis here.

You can find more details of cannabis’ interaction with common recreational drugs here.

Addiction Potential

Like any substance, cannabis can be abused, used in ways that compromise the safety and wellbeing of the individual and others.  Those who use cannabis with high frequency can develop dependence. Some studies estimate that one in ten adults become dependent on cannabis, and this risk increases to one in six for those who begin using cannabis under the age of 18 (59,60).  Signs of cannabis dependence include difficulties in meeting personal obligations or participating in important life activities, preferring to use cannabis instead.

Withdrawal-like symptoms for those individuals that form a cannabis habit are  rare, and if they do occur are relatively mild (nausea, restlessness, poor sleep, irritability).  These symptoms subside within two to three weeks (61).

Long-Term Effects

Regular cannabis use under the age of 25 (and particularly under the age of 18) can cause long-lasting changes to the brain, such as a decreased IQ, impaired memory, and other cognitive deficits due to the plasticity of the brain at this age (62).

A study reported that individuals that met the criteria for cannabis dependence showed a dampening of the dopamine reward system of the brain and an increase in negative emotion (63).  It is unknown if these changes are permanent, or if the dopamine system can return to normal function after stopping cannabis exposure.

Harm Reduction Measures

• Know why you are taking cannabis.
• If it’s your first time using cannabis, start slow. You can always take more, but you can never take less. Keeping this philosophy in mind, with any substance you consume, will minimize unnecessary harm.
• Use cannabis in a safe and comfortable environment.
• Do not underestimate the effects of cannabis. Cannabis is relatively safe, but it has the potential to cause profound psychotic effects, especially when consumed as an edible or in a concentrated form such as hash.
• Consuming cannabis with other psychoactive substances can exacerbate their effect.
• Cannabis oil in vape cartridges can contain toxic chemicals, or be contaminated by metals leaching from the cartridges themselves. Smoking this tainted oil can result in serious health effects.  Caution is advised when using cannabis vape cartridges.