Smoking weed as a teenager doesn’t negatively impact thinking and reasoning, and light use is actually associated with slight improvements in decision-making skills, according to new research.
“Decision-making skills in light cannabis users improved from age 14 to 19 while the control group performance remained approximately the same,” reads the largest longitudinal European multi-centre study on cannabis use, published in the journal Cognitive Development.
The study authored by researchers from the Charité – Universitätsmedizin Berlin and other renowned neuroscience research centres tested two hypotheses involving the effect of cannabis on cognitive abilities involved in learning, concentration, memory and processing of information.
The first hypothesis compared the neurocognitive abilities of light and heavy cannabis users in their teens to non-smokers. The second focused on the neurocognitive differences between teenagers who started smoking weed before or after age 16. “Neurocognitive” refers to the physical structures in the brain associated with various functions of thought.
“In summary, we find no evidence to support the presumption that cannabis consumption leads to a decline in neurocognitive ability,” reads the study.
The study suggests that further investigations are necessary before implications for clinical practice or policy can be made. Researchers note several other studies that came to different conclusions, like cannabis causes more impulsive decision-making behaviours, were not controlling for other drug use, and the sample sizes were small.
“Therefore, additional longitudinal studies with bigger sample sizes of heavy users and users without concurrent alcohol and tobacco use as well as longer timespans are needed to provide accurate information about the consequences of cannabis use.”
Read more: Heavy drinking, not pot smoking, associated with thinner pre-frontal cortex: study
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‘No evidence of effects’ on cognitive ability
Results found that teenagers who smoked weed, particularly those that had a late onset of cannabis use, scored higher than straightedge teens on a follow-up Cambridge Gambling Task test.
“Other than that, after controlling for confounders, we found no evidence of effects of cannabis on the remaining neurocognitive variables such as attention, working memory, short-term memory and risk-taking,” reads the study.
Researchers found that people who used cannabis heavily also used more drugs and alcohol, and started consuming cannabis earlier than light users. Earlier onset was also associated with more lifetime use.
For the study, 804 adolescents were sampled from the IMAGEN project, a multi-centre study conducted across Germany, England, Ireland and France. It examines how biological, psychological and environmental factors during adolescence can influence brain development and mental health.
Participants had an average age of 14 when the first data was collected and 19 for the follow-up collection.
Teenagers were divided in three groups: teens who had never used any illicit drug; a light cannabis user that smoked less than 20 times during the last month and heavy users who smoke more than 20 times per month.
To test the second hypothesis, researchers divided participants based if they used cannabis for the first time before age 16 or after.
Participants’ ability to think and reason was measured using numerous tests including the Cambridge Neuropsychological Test Automated Battery, which assesses overall cognitive function, and Pattern Recognition Memory, which measures visual short-term memory.
Researchers also measured IQ, substance use and socioeconomic status but found no compelling results.
Among the research subjects, men and citizens of Paris were overrepresented in the cannabis-consuming groups.
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