Rationale: There is rising concern, especially for young users of cannabis, that the drug may impair neurocognition. Neurocognition includes problem solving, abstract reasoning, language production and interpretation, and attention, which are core to brain function.

Considering that adolescence is a period for experimentation with substances and critical brain development, experts are concerned about the long-lasting impacts on cognition. 

Prior studies on the effects of cannabis on neurocognition could not eliminate the possibility of pre-existing differences between cannabis users and non-users. This longitudinal study was conducted across adolescents aged 9 to 17 to look at cannabis’ impact on neurocognition across time.

This study also controlled for other substance use, family history, and sociodemographics. 

Method: Researchers followed 11,868 subjects from age 9 to 17. Participants were tested at initiation and every 2 years with urine, oral fluid, breath, and hair samples. At each interview, the assessments consisted of self- and parent-report surveys, biosample collections, clinical mental health interviews, and substance use interviews.

Results: After the age of 14-15, cannabis users performed worse in long-delay memory, visuospatial memory, reading aloud and recall, processing speed, receptive language, episodic memory, and inhibitory control. Cannabis users displayed weakened working memory at 17 years old. 

Meaning: Cannabis is associated with the deterioration of many types of memory, language, and impulse control. These changes were observed over time, pointing to their lasting impact on overall cognition.