Using cannabis 'just 5 times' as a teen linked to increased psychosis risk

'Smoking cannabis just 5 TIMES as a teenager raises the risk of psychosis, reveals 'worrying' study' the Mail Online reports

'Smoking cannabis just 5 TIMES as a teenager raises the risk of psychosis, reveals 'worrying' study' the Mail Online reports

"Smoking cannabis just 5 times as a teenager raises the risk of psychosis, reveals 'worrying' study," reports the Mail Online. A new Finnish study followed people from the age of 15 to 30 with the aim of investigating the association between the level of cannabis use in adolescents and subsequent psychosis.

Cannabis is thought to be the most widely used illegal drug in the UK. There is growing evidence that use of cannabis, especially the potent form of herbal cannabis known as "skunk", makes people more susceptible to developing mental illnesses such as depression and psychosis (where a person is unable to tell the difference between reality and their imagination).

However, it's hard to tell the direction of the relationship. It could be the case that some young people with pre-existing mental health issues may turn to cannabis as a coping mechanism.

The study found the risk of psychosis was higher in individuals who had used cannabis 5 times or more in adolescence. However, very few people in the study used cannabis this frequently; only 66 – which accounted for around 1% of the people taking part in the study. It should be noted that analyses based on few people are less reliable.

The study is unable to confirm cause and effect as it's still difficult to say that cannabis use has caused psychosis and hasn't been influenced by other personal or lifestyle factors.

Cannabis has been linked to both mental and physical health conditions. Read more about the potential health risks of cannabis use.

Where did the story come from?

The study was led by a team of researchers from the University of Oulu in Finland. It was funded by grants from the EU and several other institutions, including: the Academy of Finland, Jalmari and Rauha Ahokas Foundation and the Northern Finland Health Care Support Foundation.

The study was published in the peer-reviewed Journal of Adolescent Health. It's available on an open access basis and is free to read online.

Both the Mail Online and The Sun chose to focus on the fact that smoking cannabis "just five 5 times" increased a teenager's risk of psychosis. But neither report recognised the limitations of the study and the fact that it was investigating any cannabis use in adolescence and "5 times or more" just happened to be the only level where a significant link with psychosis risk was identified.

What kind of research was this?

This was an analysis of data from a long-term cohort study investigating the association between cannabis use in adolescents and psychosis in later life.

Previous scientific research has pointed towards a link between level of cannabis use and the risk of psychotic symptoms. The researchers wanted to look into this further, taking account of other factors that may be influencing the link (confounders).

Cohort studies like this one are useful for better understanding the relationship between exposure and outcome. However, they still can't confirm cause and effect.

A randomised randomised controlled trial would be the best study design to look at this question, but clearly wouldn't be ethical. Therefore, a well-designed cohort study that tries to account for confounding factors is the next best option.

What did the research involve?

The researchers used the Northern Finland Birth Cohort Study (NFBC) 1986 which is an ongoing study of 9,432 children born in 2 northern provinces in Finland. Data collection has continued since.

Of these individuals, 7,344 children took part in follow-up in 2001-02 when they were 15-16 years old when substance use was questioned. One questionnaire asked the participants a range of questions such as their smoking habits, alcohol use and illegal substance use. Participants were asked "Have you ever used marijuana or hashish?" and were given the following options:

  • never
  • once
  • 2-4 times
  • 5 times or more
  • regularly

They were also asked to complete a self-reported questionnaire about symptoms suggesting they may have propensity to a psychotic disorder. For example, they were asked if they had experienced feelings that something strange is taking place within themselves or the environment, or feeling that they were being followed or influenced in a special way.

The researchers also collected information on family structure, place of residence, socioeconomic status of the family and history of psychosis in parents.

The final sample included 6,534 individuals for whom psychosis diagnosis by the age of 30 years was obtained from national registers. They looked at the effect of adolescent cannabis use and the risk of later psychosis.

What were the basic results?

Out of all the individuals, 375 reported previous use of cannabis in adolescence and 66 of these (1% of the total sample) had used cannabis more than 5 times. Daily smokers were more likely to use cannabis (22%) compared with non-daily smokers (3%).

With full adjustment for early symptoms of psychosis, smoking/alcohol use and parental psychosis, risk of adult psychosis was only increased among the 66 individuals who'd used cannabis 5 or times more in adolescence (hazard ratio 3.02, 95% confidence interval 1.14 to 7.98). Links for use less than this were not statistically significant.

How did the researchers interpret the results?

The researchers concluded: "Cannabis use at the age of 15–16 years was associated with subsequent psychosis diagnosis, and this was evident in the group with heaviest cannabis use even after controlling for baseline prodromal [initial] symptoms, daily smoking, frequent alcohol use, other substance use and parental psychosis. We found a dose–response effect suggesting that more frequent cannabis use is associated with a greater risk for psychosis."


This study benefits from using a Finnish Birth Cohort Study that has collected follow-up data for a large number of children and adolescents through to adult life. It also made attempts to account for family history and signs that the adolescent may have a propensity to developing the condition.

This is to try and address the common problem of causality (the relationship between cause and effect) to use of cannabis. That is, does cannabis use directly increase risk of psychosis, or are people with a propensity to developing psychosis more likely to use cannabis?

Unfortunately, the study can't move us much further forwards in understanding the relationship between psychosis and cannabis use. It found that the risk of psychosis was only higher in people who had used cannabis 5 times in adolescence. But only 66 individuals in the study used cannabis this frequently. Analyses based on few people are less reliable – as is evident by the wide confidence intervals around this risk figure. There was no link for less frequent use.

Other limitations include that cannabis use was self-reported in the questionnaire, meaning there is a chance that use was under- or over-reported – which may also be the case for smoking or alcohol use. The researchers also raise the point that individuals from single parent families and urban areas were less likely to participate in this study and these individuals are more likely to use cannabis. Therefore, the proportion of individuals using cannabis may be further underestimated. Finally, we don't know whether characteristics and behaviours of people in Finland directly apply to other countries and cultures.

Overall, this study is of interest and adds to the body of research in this area. As the study authors rightly point out, there is need for focused interventions helping to educate adolescents and prevent cannabis use. It may be thought of as one of the less dangerous illegal drugs, but it has been linked to mental health conditions, such as psychosis and schizophrenia, as well as physical conditions, such as bronchitis and lung cancer.

Article Metadata Date Published: Tue, 13 Mar 2018
Author: Zana Technologies GmbH
NHS Choices