Cannabis is a psychoactive drug derived from a genus of plants of the same name.  Cannabis is most commonly available in Ireland in the form of herbal cannabis (“weed”, “grass”) or cannabis resin (“hashish” or “hash”). The majority of cannabis use (97%+) is via smoking¹.

The cannabis plants contain over one hundred different cannabinoids. Cannabinoids act on the cannabinoid receptors in the endocannabinoid system mimicking the body’s endogenous endocannabinoids. The two most well researched cannabinoids are delta- tetrahydrocannabinol (THC) and cannabidiol (CBD).

The two cannabinoids act quite differently. THC has high affinity for the cannabinoid receptors in the brain (CB-1). CBD acts on the cannabinoid receptors found in the peripheral nervous system, vascular and the immune system (CB-2). THC is the psychoactive substance in cannabis which is responsible for the “high” and can lead to dependence. THC is a psychotogenic substance and cause hallucinations and paranoid thoughts and agitation. THC is also associated with impairment of cognition and thinking both in the short term and the long term. CBD is not thought to be associated with dependence or psychosis and has a mildly sedating effect.

Cannabis with THC content greater than 10% is called “high-potency” cannabis. The international and European trend shows that THC content in both herbal cannabis and resin is increasing, ^3,4. The THC level of cannabis seized in Ireland is not routinely measured but is expected to be in line with European trends. In 2010  samples of cannabis in Ireland were tested and the THC content ranged from 1 – 16%⁵. Evidence from the US indicates that potency of cannabis products increased following legalization and prices dropped⁶.

The adverse psychoactive effects of cannabis are attributed to the THC component. Studies show the higher the THC content the greater the risk of psychosis, agitation and anxiety developing⁷.

For the majority of users the effects of cannabis intoxication are acute and transient. Small doses may have no effect. Many users report pleasurable effects including euphoria which accounts for its use as a drug of recreation. Undesirable experiences include anxiety and panic attacks, dysphoria, suspiciousness and paranoia, perceptual distortions, hallucinations, cognitive impairment and psychomotor impairment ⁸.

The cognitive and psychomotor effects of cannabis account for impairment of driving and operating machinery⁹. The combination of alcohol and cannabis intoxication has a synergistic effect on driving impairment which exceeds the risk of using either exclusively¹⁰.  The Gardaí began road side testing for cannabis and other substances in 2016. In 2018 cannabis was implicated in 1,205 cases of road traffic offences according to the MBRS. A recent study suggests that chronic cannabis use can impair users’ driving even after a 12 hour abstinence period¹¹.

Yes cannabis is addictive and can lead to cannabis use disorder. Stopping cannabis consumption after a period of use can also result in significant withdrawal effects (irritability, anxiety, decreased appetite, restlessness, and sleep disturbance).

The 2014/15 Drug Prevalence Survey conducted by the National Advisory Committee on Drugs and Alcohol¹ found that 1 in 5 people who use cannabis report symptoms consistent with cannabis dependence. The risk of dependence increases with earlier age of onset of use, increased frequency of use and higher potency cannabis. A cannabis use disorder is now the most common presenting problem for people seeking addiction treatment under the age of 25 years, even more common than alcohol use disorder². From 2008-2016 there was a 4 fold increase in adolescents entering the addiction service for cannabis related addiction treatment ². A review based on US data showed early use is a predictor of dependence with people who begin using cannabis before age 18 are four to seven times more likely than adults to develop a cannabis use disorder¹².

The gateway hypothesis proposes that cannabis use leads to use of other drugs such as cocaine, amphetamine and heroin. Although scientific evidence is mixed on this question a recent large study of predictors of later opioid use disorder in the US has shown that early cannabis use (before age 18) was the dominant predictor (Wadekar, 2020).

Yes. Cannabis use can precipitate new mental illness and exacerbate pre-existing mental illness.

Psychosis

Cannabis users are at 3-4 fold increased risk of  development of acute psychosis¹³ with evidence that this association is increased to 5-6 fold with early use of high potency cannabis¹⁴.  People with cannabis-induced psychosis are at high risk of progression to a chronic psychotic disorder such as schizophrenia. It has been shown that 1 in 5 people who suffer cannabis-induced psychosis will progress to a schizophrenia diagnosis within 3-4 years. (Kendler et al, 2019)

Mood Disorders

Early-onset use of cannabis is also associated with an increased risk of  development of major depressive disorder ^15,16.Cannabis use disorder has been associated with a greater risk of Bipolar affective disorder onset¹⁷ and reduced length of time between relapses.

Suicidal behaviours

Chronic cannabis users are more likely to report thoughts of suicide than non-users. The National Self-Harm Registry Ireland Annual Report 2018 states that cannabis was most common street drug used among men aged 15-24 year-old self-harm presentations – present in 8% of overdose acts¹⁸. A recent systematic reivew found that the risk for suicide attempts in young cannabis users was more than 3 times higher than in young people who did not use cannabis¹⁶.

The use of high potency (high THC) cannabis regularly and at an early age are the strongest risk factors for cannabis users. Cannabis has an effect on synaptic pruning and white matter brain development and users with a “still developing brain” are at most risk. Other important risk factors are a history of psychosis, a family history of psychosis and individual genetic vulnerabilities.

The highest risk is for users of high potency cannabis on a regular basis. However, studies have shown that even a small amount of cannabis exposure in early adolescence seems related to structural brain changes and mental illness¹⁹.  However THC has been shown to induce psychosis in healthy people with no history of mental illness The Report from the National Academies of Science, Engineering and Medicine (2017) has stated that cannabis use is associated with  psychosis:  higher the dose, the higher the risk.

Cannabis use has effects on brain structure and development²⁰.The adolescent brain seems to be most at risk from neurodevelopment and structural changes²¹. While chronic and high frequency use of cannabis is associated with structural brain differences a recent review has shown that even low exposure to cannabis during adolescence may result in changes in grey matter volume in the brain¹⁹. Larger studies and further replication is required to investigate this relationship.

Cannabis use during pregnancy is associated with low birth weight and disruption of the endocannabinoid system which may lead to neurodevelopmental problems²².

Yes, cannabis use is associated with harms for the user, their family and the society they live in. Parental cannabis use is recognised as an adverse childhood experience and a risk factor for worse health and socioeconomic outcomes²³. A severe cannabis use disorder can lead to parenting problems, and neglect of children. The HSE and TUSLA have issued a joint advisory document on addressing these hidden harms of substance misuse which has practical advice on cannabis misuse within households²⁴.  Cannabis use has also been implicated in many road traffic accidents. In the US, the states that legalised recreational cannabis sales had higher traffic fatalities in the year following legislation change²⁵. Cannabis use increases the risk of violent behaviour in individuals with psychotic disorders²⁶. Cannabis use is also implicated in family and domestic violence and  cases where children are exposed to violence^27,28.

Cannabis use can adversely affect academic and work performance and is linked to decreases in IQ of up to 8 points in those who start using cannabis at a young age ^29,30. Cannabis use decreases memory performance tasks in students³¹. Multiple studies across different sites have shown early life cannabis use, is strongly associated  with poorer educational outcomes, lower income, greater welfare dependence and unemployment and lower relationship and life satisfaction^32,33.

A comprehensive recent review found that cannabis-based products are not effective in the treatment of mental illnesses (e.g. psychosis, depressive disorders, anxiety disorders, post-traumatic stress disorder and Tourette’s syndrome), and were associated with adverse side effects³⁴. There is concern that people with mental illnesses may self-medicate with cannabis, or have it unwisely recommended to them, and this may delay both appropriate help-seeking and provision of evidence-based treatments. At time of publication cannabis and cannabis products are not evidence-based treatments for mental illness.

Cannabis smoke contains similar toxins to those found in tobacco smoke (i.e. carbon monoxide, aldehydes, acrolein, phenols and carcinogenic polycyclic aromatic hydrocarbons) ³⁵.
Cannabis use is associated with many physical harms including:

• cannabis hyperemesis syndrome³⁶,
• exacerbation of respiratory illnesses including bronchitis^37,38
• cardiovascular illness including stroke and cardiac arrhythmias
• certain types of cancer^39,40

Survey data both nationally and internationally would suggest the perception of the harms of cannabis have diminished over recent years especially among adolescents².  Current research is on-going to assess and understand this trend and the possible role of media coverage of cannabis use and effects of commercial cannabis marketing.