This well conducted prospective cohort study looks at the impact that cannabis use can have on the development of the brain over a 20 year period, from the teenage years to middle-age.

The research studied 1,037 people from New Zealand who are part of the Dunedin Multidisciplinary Health and Development study, which followed participants from birth in 1972/32 to the present day.

Intelligence was assessed using various IQ tests in childhood at ages 7, 9, 11 and 13 and again in adulthood at age 38.

Participants were interviewed aged 18, 21, 26, 32 and 38 years old and asked about cannabis use (including when they first started) and their intelligence was tested using a range of mental arithmetic, vocabulary and coloured block tests. The final test (aged 38) also involved each participant nominating a person who knew them well who also answered questions about the participant’s attention and memory.

The researchers used reliable statistical analysis methods and considered other factors (such as other substance misuse, mental illness and length of education) when measuring mental functioning.

Here’s what they found:

• People who reported never using cannabis showed a slight increase in IQ from 18 to 38 years old
• People who reported being cannabis dependant at 1-3 of the age assessment points, showed a decline in IQ (an average of 5.23 IQ points lost for the heaviest users)
• Interestingly, persistent cannabis use in adolescence was associated with greater IQ decline
• Adult cannabis users who stopped using for one year or more restored their neuropsychological functioning at 38 years, but this restorative effect was not seen in adolescent users who also stopped for one year or more

The researchers concluded that persistent cannabis use over a 20 year period is associated with neuropsychological decline and greater decline is seen among persistent users. They propose that the not yet fully developed adolescent brain is more susceptible to damage from cannabis.

But:

• It’s important to remember that cohort studies such as this can never prove a cause and effect link between cannabis and intelligence. There may be other factors at work (e.g. unmeasured mental health problems or other socioeconomic factors) that influenced the results
• The study asked people how much they smoked and when they started, which may lead to inaccuracies in the results
• It’s very difficult to measure how much people smoke, especially with the increased strength of cannabis in recent years. The increased potency of the drug may actually make the effects on IQ even more damaging

Link

Meier MH, Caspi A, Ambler A, Harrington H, Houts R, Keefe RS, McDonald K, Ward A, Poulton R, Moffitt TE. Persistent cannabis users show neuropsychological decline from childhood to midlife. Proc Natl Acad Sci U S A. 2012 Aug 27. [Epub ahead of print] [PubMed abstract]