Mental health

Does smoking 'dope' turn you into one?

“Adolescents who are regular users of cannabis are at risk of permanent damage to their intelligence, attention span and memory,” reported the Guardian.

The news was based on an impressive and wide-ranging study of 1,037 New Zealanders who were followed from birth up to the age of 38.

Researchers aimed to investigate the association between persistent cannabis use and mental function over a 20-year period, and to see whether greater decline was seen among those who started using cannabis in their teens. They found that those who did and then carried on using cannabis into later life experienced a small drop in IQ of a few points. They also scored less than non-cannabis smokers in other tests of mental function, such as mental arithmetic.

Interestingly, other studies have not found a similar drop in IQ or mental function in people who begin smoking cannabis as an adult. One possible theory to explain this is smoking cannabis as a teen could disrupt the development of the brain (the brain is not fully developed until around the age of 18). This in turn could lead to corresponding problems with mental functions. Further research is required to confirm or disprove this theory.

While the evidence is compelling, as the researchers admit, there is still not enough of it to show a clear direct cause and effect between teenage cannabis smoking and reduced intelligence. The possible observed link could be due to other unmeasured factors (for example, other mental health issues).

Overall this study provided some evidence to support the growing literature on the potential harm of cannabis, particularly among adolescents. As one researcher was quoted saying: ‘Cannabis... is risky for under-18 brains’.

Where did the story come from?

The study was carried out by researchers from the University of Otago in New Zealand, Duke University in the US, the Kings College in London and other institutions. It was supported by the New Zealand Health Research Council, the UK Medical Research Council, the US National Institute on Aging, the US National Institute of Mental Health and the US Institute on Drug and Abuse.

The study was published in the peer-reviewed journal PNAS (Proceedings of the National Academy of Sciences).

The story was covered appropriately by BBC News and picked up by a variety of other papers and online media.

What kind of research was this?

This was a prospective cohort study looking at the effects of cannabis use on IQ in New Zealand.

Cohort studies are useful for looking at possible associations between various lifestyle factors (such as smoking cannabis) and health outcomes (such as a person’s neuropsychological development). They enable researchers to follow large groups of people for many years but they cannot establish cause and effect.

A prospective study recruits appropriate participants and looks at the exposures or provides treatments, and then measures outcomes of interest in these people over the following months or years.

Results from prospective studies are usually considered more robust then retrospective studies, which either use data collected in the past for another purpose, or ask participants to remember what has happened to them in the past.

The difficulty with a cohort study such as this, is that it cannot take into account all the possible factors that could be related to both cannabis use and mental functioning. So there may be other factors that are missed by the researchers – as the paper states – ‘there may be some unknown ‘third’ variable that could account for these findings’.

What did the research involve?

Researchers recruited 1,037 individuals from the larger Dunedin Multidisciplinary Health and Development study in New Zealand that was investigating the long-term health and behaviors of participants. The study participants were followed from their birth in 1972/1973 up until the age of 38.

Cannabis dependence was determined using recognised criteria in interviews at five different ages:

  • 18 years – people at this point were also asked about any earlier history of cannabis use
  • 21 years
  • 26 years
  • 32 years
  • 38 years

Cannabis dependence is normally defined as:

  • experiencing withdrawal symptoms if the supply of cannabis is withdrawn, or conversely, smoking cannabis to avoid these symptoms
  • being unable to control or cut down the amount of cannabis that you smoke
  • developing an increasing tolerance to the effects of cannabis

In assessing persistent cannabis use, participants were grouped as those who:

  • never used cannabis
  • used cannabis but not regularly
  • used cannabis regularly at one of the age assessment points
  • used cannabis regularly at two of the age assessment points
  • used cannabis regularly at three or more age assessment points (this was considered as persistent cannabis dependence)

To determine neuropsychological functioning, intelligence was assessed using various IQ tests in childhood at ages 7, 9, 11 and 13 and again in adulthood at age 38.

As well as the standard IQ test, other tests of mental functioning were also carried out, including:

  • mental arithmetic
  • vocabulary testing
  • block design test (where people are asked to assemble coloured blocks into a set pattern) 

At the 38-year mark, participants also nominated a person who knew them well (who the researchers called informants).

These informants were asked to fill out questionnaires about the person’s mental functioning including any attention and memory problems.

The researchers then looked at changes in IQ from childhood to adulthood to see if cannabis had an effect on any changes seen.

The researchers analysed their results using statistical methods and took into account other factors that could account for the decline in mental functioning such as:

  • alcohol and tobacco dependence
  • other drug use (for example, heroin, cocaine and amphetamines)
  • a diagnosis of schizophrenia
  • number of years spent in education

What were the basic results?

The main findings from this study were:

  • Participants who reported more persistent cannabis use showed greater neuropsychological decline. For example, those who reported never using cannabis  showed a slight increase in IQ, whereas those considered cannabis dependent at one, two or three of the age assessment points, showed declines in IQ.
  • Participants with more persistent cannabis dependence generally had greater neuropsychological impairment.
  • IQ impairment was more pronounced among those who used cannabis in adolescence, with more persistent use associated with greater IQ decline.
  • Adolescent cannabis users (infrequent and frequent use), who stopped using cannabis for one year or more, did not fully restore their neuropsychological functioning at 38 years, whereas participants who were frequent or infrequent cannabis users in adulthood did. 

How did the researchers interpret the results?

The researchers conclude that persistent cannabis use over 20 years is associated with neuropsychological decline and that greater decline is seen among those who are more persistent cannabis users.

They say this effect was most evident among those who take up cannabis use while in adolescence. The researchers theorised that this could be the result of persistent cannabis use during teenage years disrupting the development of the brain.

In discussing their research findings the authors say that, ‘prevention and policy efforts should focus on delivering to the public the message that cannabis use during adolescence can have harmful effects on neuropsychological functioning’.

They went on to add that a useful harm reduction message arising from the research was that, (to paraphrase) ‘ideally people should avoid smoking cannabis, but if they are determined to do so, at least wait until adulthood’.

Teenagers who are currently smoking cannabis should be encouraged to quit.


Overall, this study provided some evidence to support the growing literature on the potential harm of cannabis, particularly among adolescents.

The most important limitation is that, despite the author’s efforts to adjust for confounders, it is always possible that other factors (for example, socioeconomic factors or other unmeasured mental health issues) influenced the results and were underlying the apparent association. It is important to note that this research does not prove that there is a direct causal link (that is, teenage cannabis use leads to an IQ decline) only that there is an association.

It’s also worth noting that this research undertook extensive statistical calculations to look at the relationship between different measures of IQ and different duration of cannabis use, some of which involved only small sample sizes. For example, despite the large initial sample size (1,037), only 41 people (3.95% of the people surveyed) used cannabis regularly at all three time points. Calculations based on such small sample size decreases the reliability of these risk associations.

Another issue is whether cannabis use was accurately recorded. Interestingly only seven participants reported trying cannabis by age 13, and cannabis use during adolescence was only accounted for in the assessment at 18 years where participants were asked about use in the previous year, that is, while they were 17. It is also possible that participants did not accurately report their cannabis use patterns in the year before each of the assessments, which can make the results less reliable.

The fact that informants were also asked to assess participants on things such as memory and attention span may also make the results somewhat unreliable as people’s personal opinions are by definition highly subjective.

Validation of cannabis use using laboratory measures would have made the results more reliable. However, persuading ‘dope-smoking 20-somethings’ to attend regular blood tests may be somewhat challenging.

Another factor is that the potency of the cannabis smoked was not reported. There has been a growing trend of people smoking stronger strains of cannabis (such as skunk) over the course of the last few decades.

So if there is a dose-dependent effect between cannabis use and IQ impairment then the effect in today’s teenagers could be even more pronounced.

The researchers note that further research is needed on the effects of the quantity, frequency and the age-of-onset of cannabis use on neuropsychological impairment.

The full longer effects of cannabis use are not completely known, but in the shorter term cannabis can cause variable psychological effects, which may differ extensively between individuals.

Despite these limitations this is a useful piece of research that adds to a growing body of work that suggests persistent cannabis use at a young age can have a negative impact on mental health and mental functioning.

Analysis by *NHS Choices. Follow Behind the Headlines on twitter*.

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