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What Are The Harms of Cannabis Use?


No drug is without risk of harm, this includes Australia’s most widely used illicit drug cannabis.

The following outlines the relative risk of cannabis, harms associated with its use and how prohibitionist policy causes a number of harms.

How Harmful Is Cannabis?

Compared to other common drugs, including alcohol and tobacco, cannabis is much less harmful on average.

In 2010, a multicriteria analysis by the UK Independent Scientific Committee on Drugs was conducted to assess the harms of 20 drugs based on 16 criteria.

The analysis has been very influential on drug policy. It found that heroin, crack cocaine, and methamphetamine were the most harmful drugs to individuals and alcohol, heroin, and crack cocaine were the most harmful to others. Cannabis was considered to be moderate-to-low risk across all criteria.

A more recent study from Australia found similar results. Cannabis ranked 13th out of 22 drugs assessed (Figure 1).

Figure 1: Harm to self and others of selected drugs (from Bonomo et al., 2019)

Harms Associated With Cannabis

Acute health harms as a result of cannabis use, such as overdose, are very rare.

Harms commonly associated with recreational use of cannabis are generally mid to long term, including dependence, cognitive deficits, mental health problems, and, among younger people, poor school outcomes.

The causal direction of these harms is not well established and the evidence of association is mixed. It may be, for example, that people with existing cognitive deficits or other risks factors for developing cognitive deficits are more likely to use cannabis, rather than cannabis causing those cognitive deficits.

There is no doubt that cannabis can produce short-term changes to thinking, working memory, executive function, and psychomotor function – that’s one of the reasons why people use it.

A large longitudinal cohort study found that, among people who were heavy users of cannabis, cognitive functioning declined more than people who used occasionally, but more recent longitudinal twin studies concluded that there is no reliable relationship between cannabis use and cognitive functioning. The studies showed that adolescents who used cannabis had lower cognitive functioning before they started using cannabis.

Regular cannabis use has been found to double the risk of schizophrenia, but risk of psychosis due to cannabis consumption appears to be limited to certain vulnerable populations, who may have gone on to develop schizophrenia without cannabis use.

Young people, individuals with previous experience of psychosis and those with a high genetic risk of schizophrenia appear most at risk of psychosis due to cannabis consumption. However, there are no indications that higher rates of cannabis consumption in the population increases the rate of schizophrenia.

In addition, to put the risk in context, the rate of schizophrenia in the general community is approximately 1%. Therefore, the rate of schizophrenia among people who use cannabis is potentially 2% of the less than 12% that use regularly.

Teen cannabis use is associated with poorer school outcomes but, again, causation has not been established. It is likely that social determinants of health outcomes including family poverty, education, and employment are drivers of both early cannabis use and poorer school outcomes.

The lifetime risk of developing cannabis dependence is much lower than for other illicit drugs at 9% compared to 23% for heroin, 17% for cocaine, 15% for alcohol and 11% for stimulants.

Daily use increases risk of dependence with around a third of daily users thought to be dependent. Around 37% of people who use cannabis are daily users. So, one third of 37% of all users are at risk of dependence.

Claims that cannabis is a ‘gateway drug’ which leads to consumption of more harmful drugs, has been thoroughly debunked.

While it is true that people who use other drugs later most often use cannabis first, the converse is not true – most people who use cannabis do not go on to use other drugs. In addition, alcohol and tobacco usually precede cannabis use, which if the theory were correct, would make those drugs the ‘gateway’ to cannabis and other drug use.

Harms Of Cannabis Prohibition

Australia’s official policy on alcohol and other drugs, the National Drug Strategy, is based on harm minimisation.

There are three pillars to this policy, including supply reduction (law enforcement), demand reduction (prevention and treatment) and harm reduction.

In a harm minimisation framework, policies should focus on keeping harms to a minimum, resulting in a net reduction in harm. However, some of the most significant harms from using illicit drugs, including cannabis, are precisely because they are illegal.

Despite the platform of harm minimisation, the focus on prohibition to achieve this has resulted in:

Recognising the failures of prohibitionist approaches to drug policy, many countries are now exploring other strategies to reduce cannabis related harms, including decriminalisation and legalisation.