A new study from researcher in the U.S. and New Zealand says cannabis use causes harms to intelligence, memory, and attention in people who start using marijuana before age 18 and continue using it through the years. The harmful effects continue even after use is discontinued in mid-life, say the researchers.
Researchers from the Dunedin Multidisciplinary Health and Development Research Unit, Department of Preventive and Social Medicine, School of Medicine, University of Otago in New Zealand along with the Developmental Psychiatry Centre, Institute of Psychiatry, King’s College London, and colleagues from Duke University in North Carolina followed 1,037 individuals from from birth to age 38 and interviewed them at ages 18, 21, 26, 32, and 38 to ascertain cannabis use.
Neuropsychological tests were conducted at age 13, before marijuana use, and again at age 38 after a pattern of persistent marijuana use had developed.
An average drop of 8 IQ points was seen on intelligence tests for pot users. With a normal mean of 100 IQ points being at the 50 percentile, a drop of 8 points to 92 would drop the individual to the 29th percentile, significant drop.
Allowing for education levels, there were broad neuropsychological declines and cognitive problems for persistent cannabis users. The more the use, the greater the decline. Cessation of cannabis use did not fully restore the neuropsychological functioning to normal.
The researchers conclude the findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents.
The study was published this week in the Proceedings of the National Academies of the Sciences titledPersistent cannabis users show neuropsychological decline from childhood to midlife.
(photo: marijuana plants at Hendry county grow house - Courtesy Hendry Sheriff's Office)