a young person's guide to


and your mental health


Cannabis is a complicated plant and it has the potential to affect our brain development up until the age of 25.


Based on interviews with Irish psychiatrists, this website considers the risks posed by cannabis to your mental health.

The chemistry

Cannabis is a complicated plant but a lot of the debate around cannabis use is related to 2 particular compounds:

  • THC (Delta-9-TetraHydroCannabinol)

  • CBD (Cannabidiol)

THC is the psychoactive component, and it is what gets you high. However, it can also cause side effects such as psychosis.

CBD is being investigated as a medicine, while a wide range of claims are made, its benefits are currently only proven for a small number of conditions such as certain types of epilepsy.


Watch our experts Dr Bobby Smyth and Dr Aisling O'Neill discuss this in more detail. 



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Changes in strength

The level of THC in the cannabis people are using has risen significantly in recent years. The THC content of marijuana samples seized by the Drug Enforcement Administration (DEA) in the United States has climbed from around 3% in 1980 to 12% in 2012 and recently to 20%. Some cannabis edible products can have 90% THC content. Similar increases have been observed in Ireland.

As a rough comparison, this is the equivalent of changing from drinking a half pint of light beer to a half pint of vodka.

Changes in cannabis strength

Changes in cannabis strength

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Concerns for young people

The risks of cannabis use for young people fall into three main categories:


Cannabis is popularly portrayed as non-addictive however the rise in THC levels have led to an increase in dependency on cannabis. Dr Bobby Smyth from HSE Addiction Services estimates that about 1 in 6 adolescent users will become dependent on (or ‘addicted to’) cannabis.


One of the biggest areas of concern is the risk of cannabis induced psychosis. Psychosis is ​a mental health condition where the person loses touch with reality. They may see, hear or feel things that are not actually there. They may also have beliefs that have no basis in reality such as paranoia and this can lead to violent behaviour. Their thinking may become disorganised.

Psychosis was less common with low strength  cannabis but it is becoming more and more of a concern as higher strength cannabis becomes available. Heavy users of strong cannabis are 5 times more likely to develop a psychotic disorder. Across Europe it is estimated that about one in every five new cases of psychosis are caused by cannabis.


Cognitive Impairment

Our brains are continually developing until about 25 years of age. The chemicals in cannabis target parts of the brain that control this development so there is concern that regular use can disrupt the natural brain development process.


Studies have shown a negative impact on thinking and memory from cannabis use. Some of these negative effects disappear in 2-3 weeks but some appear to persist and may become permanent. Studies have shown that young people who use cannabis regularly can experience a drop in overall intelligence scores which may be irreversible.

Cannabis: Concerns for Young People

Cannabis: Concerns for Young People

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Effect on existing mental illnesses

Many people with existing mental problems like anxiety or depression can be drawn to substances like cannabis to manage the way they are feeling. While this may work in the short-term, the news is not good in the long term. 

Our experts Dr Bobby Smyth and Dr Aisling O'Neill discuss the problems that substance abuse such as cannabis can cause to your mental health.

Effect of Drug Use on Existing Mental Illness

Effect of Drug Use on Existing Mental Illness

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Psychosis and cannabis - which came first?

Several well-respected research studies have shown a link between cannabis use and psychosis. But how do we know whether the cannabis use caused the psychosis or whether people with psychosis are more likely to use cannabis?

Our expert Dr Bobby Smyth explains how we answer these kind of problems in scientific research.