Cannabis and Mental Health: Frequently Asked Questions

For Medical and Health Professionals

This Fact sheet is aimed at medical and health professionals and aims to address 20 Frequently Asked Questions about cannabis. Cannabis and cannabis-based products are becoming more widespread within Ireland. People who use the mental health services, both children and adults are at higher vulnerability to the harms of cannabis use therefore Irish mental health professionals should be familiar with these risks especially as the perception of cannabis as a harmless substance and the amount of misleading information about cannabis has increased in recent years.

  • Cannabis is the mostly commonly used illegal drug in Ireland
  • Cannabis is easily available in Ireland including high potency versions
  • Cannabis adversely affects mental health
  • Young people who use high potency cannabis regularly are at highest risk of harms
  • Cannabis can be additive
  • Cannabis and cannabis products are not evidence-based treatments for mental illness
  • Cannabis use is associated with many social and educational problems

View this information sheet as a PDF here.

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.

Cannabis is the most widely used illegal drug in Ireland. The National Advisory Committee on Drugs and Alcohol (NACDA)1 conducted a general population study in 2014/2015 and found  1in 4 of respondents had used cannabis at least once in their life time with 6.5% of respondents reporting cannabis use within the last month. A a study2 using the National Psychiatric In-Patient Reporting System (NPIRS) data found that cannabis-related admissions had increased by 140% between 2011 and 2017 for people aged 15-34.  The European School Survey Project on Alcohol and Other Drugs (ESPAD) study in 2015 has reported that 10% of 15/16 year olds had used in the past month. The average age of initiation of cannabis use was 14 years.

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.


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 violence27,28.

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 satisfaction32,33.

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 bronchitis37,38
  • cardiovascular illness including stroke and cardiac arrhythmias
  • certain types of cancer39,40

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.

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