Cannabis Use In Teens Used To Link Drop In IQ

New research finds that persistent cannabis use during adolescence can cause lasting harm to a person's intelligence, attention and memory.

Researchers from the Medical Research Council (MRC) Social Genetic and Development Psychiatry Centre at King's College London's Institute of Psychiatry (IoP) collaborated with scientists from Duke University in the USA and the University of Otago in New Zealand on the study published today in Proceedings of the National Academy of Sciences. They found that individuals who started using cannabis in adolescence and used it more than once a week for years afterwards showed an average decline in IQ of 8 points when their age 13 and age 38 IQ tests were compared.

The team studied the association between persistent cannabis use and neuropsychological functioning in a group of 1,037 individuals (from the Dunedin Multidisciplinary Health and Development Study) followed from birth in 1972/1973 to age 38. Cannabis use was ascertained in interviews at ages 18, 21, 26, 32 and 38. Neuropsychological testing to assess memory, processing speed, reasoning and visual processing was conducted at age 13, before cannabis use, and again at age 38.

About 15 percent of the study group were considered persistent cannabis users and 5 percent were using cannabis more than once a week before age 18. Individuals who started using cannabis persistently during adolescence a greater decline in neuropsychological function that those who started in adulthood.

Persistent cannabis use was associated with broad impairment across five domains of neuropsychological functioning, and remained significant even after controlling for years of education and use of other drugs including alcohol. Quitting or reducing cannabis use did not fully restore neuropsychological functioning among individuals who started using cannabis in adolescence.

Professor Terrie Moffitt, lead author, who holds a dual appointment at King's and Duke said: "This work took an amazing scientific effort. We followed almost 1000 participants, we tested their mental abilities as kids before they ever tried cannabis, and we tested them again 25 years later after some participants became chronic users. Participants were frank about their substance abuse habits because they trust our confidentiality guarantee and 96% of the original participants stuck with the study from 1972 to today. It's such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains."


Related UK National News Stories
Click here for the latest headlines.

02 March 2011
Cannabis Use Precedes The Onset Of Psychotic Symptoms In Young People 

Cannabis use during adolescence and young adulthood increases the risk of psychotic symptoms, while continued cannabis use may increase the risk for psychotic disorder in later life, concludes a new study published on bmj.com.
11 August 2014
Cannabis Factory Uncovered In Cardiff Hospital
Police in Cardiff searching for a missing person have uncovered a cannabis farm in a disused ward of a psychiatric hospital. It is understood that around 30 cannabis plants were uncovered by officers during a search at Whitchurch Hospital in Cardiff. News of the cannabis farm was revealed in a report by the BBC.
12 September 2012
Boom In Cannabis Trade Leading To Increase In Gang Violence
A senior police officer has warned of an upsurge in shootings and gang-related violence related to a ‘boom’ in the cannabis trade.
21 September 2009
NI Teen Drug Use 'Highest In Europe'
There's been a surprise development on drug culture. Around one in 10 children in Northern Ireland under the age of 12 have used cannabis, according to research conducted by Queen's University. The Youth Development Survey questioned 4,000 teenagers in 43 schools in Northern Ireland about drug use.
07 May 2008
'U-Turn' As Cannabis Reclassified To Class B
Cannabis has been reclassified as a Class B drug, it has been announced today. In making her decision The Home Secretary Jacqui Smith went against the advice of The Advisory Council on the Misuse of Drugs (ACMD), which wanted to keep it a Class C drug. The council has not been overruled for 30 years.