Does teenage cannabis use lower intelligence?


Although in recent years the number of young people (16 – 24 yrs old) using cannabis has declined, cannabis remains the most commonly used illicit drug, with up to 1 in 6 young people having used the drug in the last year (Home Office 2015).

Many things can potentially have an impact on young people who use drugs; amongst them are the short- and long-term effects on IQ and education outcomes, which have been previously explored in a range of studies. Commenting on new research back in 2014 one Elf said ‘…regular cannabis use before age 17 is linked with lower high school completion and degree attainment.’ (Tomlin, 2014)

So two years on, we have new research that examines the effect of early cannabis use on IQ and educational outcomes (Mokrysz et al, 2016).

1 in 6 young people use cannabis

1 in 6 young people use cannabis.


The authors used data from some participants of a larger cohort study. Out of the 4,621 participants in the cohort, 2,235 participants had sufficient data to conduct an analysis of the variables the researchers needed.

Information about use of cannabis was collected via a self-reported questionnaire when the participants were aged 15. Five categories of cumulative cannabis use were included, ranging from never used to used more than 50 times.

The participants IQ was measured at age 8 then again at age 15. Educational ‘key stage’ data including GCSEs was collected for the participants when they were aged 11 and 16.

Potential confounding factors were considered and data collected relating to the participants:

  1. Mothers use of substances, mental health and educational achievements
  2. Participants history of mental health
  3. Participants self-reported use of cigarettes, alcohol, ketamine, LSD, amphetamine, inhalants, ecstasy and cocaine.
This is a large cohort study, involving teenagers from the Avon Longitudinal Study of Parents and Children.

This is a large cohort study, involving teenagers from the Avon Longitudinal Study of Parents and Children.


When adjustments were made for confounding variables such as childhood behavioural problems, childhood depressive symptoms, other substance use and the mothers’ use of cannabis during pregnancy, cannabis use did not predict lower IQ performance or lower educational achievement.

Cannabis use was not correlated with lower IQ

Cannabis use was not correlated with lower IQ.


The authors concluded:

Our findings imply that previously reported associations between adolescent cannabis use and poorer intellectual and educational outcomes may be confounded to a significant degree by related factors.


The notion that cannabis use itself is casually related to lower IQ and poorer educational performance was not supported in this large teenage sample.

Strengths and limitations

  • The type of cannabis used by these participants was not assessed, nor was the dose. The term cannabis is often used in research without differentiating between high and low potency varieties. This makes many of the seminal studies on cannabis less relevant.
  • A strength and limitation is the period in time that this study was conducted. As higher potency forms of cannabis have overtaken lower potency forms in terms of availability and consumption in recent years, we can assume it is likely that the participants in this sample were exposed to higher potency cannabis, but we can’t be sure. This might account for some of the difference in findings compared to earlier studies on this topic.
  • Relying on self-reported drug use can be unreliable, but the authors tried to reduce this limitation by including a fictional drug ‘spanglers’ in their questionnaire and excluded 3 participants who reported using it.
  • Data was incomplete for 2,386 individuals in the original sample. Would this incomplete data have changed the results? This is significant as those young people who were excluded from school at the key data collection points would have been missing from this study. They are likely to be a vulnerable group and be at significant risk of problematic substance use.
  • It is inferred but not clear if these were children who attended state school, therefore passively excluding privately educated and home educated children.
  • A particular strength of this research is the way that confounding variables were considered and analysed. This goes some way to separate out the effect other substances might have influenced the outcomes measured.
  • Assessment of outcome stopped at the age of 16, so we don’t know what the effects of cannabis use on IQ and educational outcome is for this cohort as they age.
Previous research results have been influenced by confounding variables.

Previous research results have been influenced by confounding variables.


What jumps out from this study is that the majority (91%) of the sample population mix tobacco with cannabis. Exposure to tobacco at this age raises concerns in terms of health and the potential for developing dependence on a legal drug. So we might have been too distracted by illicit drugs such as cannabis and not given sufficient attention to legal albeit regulated substances like tobacco. Untangling the impact of tobacco use on IQ, educational achievements and mental health problems such as psychosis is both challenging and urgent (Jauhar, 2014).

So how does this research help us advise our children, clients or their parents who are concerned about the effects of cannabis? What would you say to yourself as a teenager? It would be good to hear your advice, so please post a response and share your thoughts with fellow Elves…

Tobacco and cannabis use are inextricably linked.

Tobacco and cannabis use are inextricably linked.


Primary paper

Mokrysz C, Landy R, Gage SH, Munafò MR, Roiser JP, Curran HV. (2016) Are IQ and educational outcomes in teenagers related to their cannabis use ? A prospective cohort study. Journal of Psychopharmacology 1–10. DOI: 10.1177/0269881115622241

Other references

Home Office (2015) Drug misuse: findings from the 2014/15 crime survey for England and Wales (PDF). (Accessed 26/1/16)

Tomlin A. Cannabis use in young people linked with lower high school completion and degree attainment. The Mental Elf, 10 September 2014:

Jauhar S. Cannabis, cigarette smoking and psychosis: do we need a rethink? The Mental Elf, 14 May 2014.

Photo credits

Share on Facebook Tweet this on Twitter Share on LinkedIn Share on Google+