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Cannabis use during adolescence and young adulthood was linked to lower academic performance, a systematic review and meta-analysis suggested.

Among 63 studies that comprised 438,329 participants, moderate-certainty evidence indicated that cannabis use among individuals ages 24 years and younger was likely associated with lower school grades (odds ratio [OR] 0.61, 95% CI 0.52-0.71) as well as less likelihood of high school completion (OR 0.50, 95% CI 0.33-0.76), university enrollment (OR 0.72, 95% CI 0.60-0.87), and postsecondary degree attainment (OR 0.69, 95% CI 0.62-0.77), reported Li Wang, PhD, of McMaster University in Hamilton, Ontario, and colleagues.

Evidence of the same certainty also showed cannabis use among this age group was likely associated with increased school dropout rate (OR 2.19, 95% CI 1.73-2.78) and school absenteeism (OR 2.31, 95% CI 1.76-3.03), they noted . Absolute risk effects ranged from 7% to 14%.

"Although our findings support a negative association between cannabis use and academic achievement, the mechanism of action is uncertain," the researchers wrote. "Cannabis-induced impairment of cognitive function and motivation may play a role; however, whether cannabis use is a cause, correlate, or consequence of these factors is inconclusive."

In their introduction, Wang and colleagues said that, in 2019, 37% of U.S. high school students reported lifetime cannabis use and 22% reported using in the past month. They also noted that the potency of cannabis has increased over time in the U.S., from approximately 4% tetrahydrocannabinol in 1995 to 14% in 2019.

"Cannabis use can lead to short-term cognitive impairments, including memory deficits and impaired attention," they said, with chronic use among adolescents "linked to long-term changes in brain architecture, resulting in impaired information processing and decreased cognitive, memory, and attentive capacity in adulthood."

"The most recent systematic review on adolescent cannabis use and academic achievement found that heavy cannabis use was associated with worse educational outcomes," they continued. "However, this review had important limitations, including the lack of statistical pooling of associations, failure to assess the risk of bias of individual studies and overall certainty of evidence, and a limited search window (2014-2019)." Their analysis was intended to address these limitations.

In the current study, subgroup analyses with moderate credibility suggested worse academic outcomes for frequent cannabis users (weekly or daily) compared with less frequent users (less than weekly), and for those who began cannabis use earlier (at 16 years or younger) rather than later (older than 16).

For instance, two studies indicated a larger association with lower school grades for more frequent cannabis users (OR 0.58, 95% CI 0.53-0.64) versus less frequent users (OR 0.72, 95% CI 0.69-0.75). And two studies indicated a larger association among cannabis users who started using earlier with lower odds of high school completion (OR 0.42, 95% CI 0.28-0.63) versus those who did later (OR 0.77, 95%CI 0.53-1.10).

Low-certainty evidence suggested that cannabis use among adolescents and young adults was also possibly associated with increased unemployment, (OR 1.50, 95% CI 1.15-1.96), with an absolute risk increase of 9%, they added.

Wang and colleagues included research from relevant databases (CINAHL, EMBASE, MEDLINE, PsycInfo, PubMed, Scopus, and Web of Science) from inception through November 10, 2023.

The median age of participants in 31 studies was 16 years, the median proportion of female participants in 47 studies was 52%, and the median proportion of white participants in 32 studies was 70%.

The pooled rate of cannabis use at baseline among 47 studies was 33%, though most studies (89%) did not report the purpose (recreational or medicinal), type, or route of cannabis use.

Concurrent substance use was 26% for tobacco or cigarettes in 21 studies, 46% for alcohol in 26 studies, and 10% for cocaine or other substances in 15 studies. Additionally, there were 12 studies that reported substance use disorder, including cannabis abuse or dependence (median proportion of 8% in nine studies), alcohol abuse or dependence (33% in five studies), and general substance use disorder (6% in three studies).

Limitations included that most studies analyzed in the review were at high risk of bias due to high variability in measures of cannabis use, study populations, geographic areas, and cannabis-related policies, the authors noted. Additionally, they were unable to study the impact of legal status or recreational versus medicinal cannabis as possible sources of variability for most outcomes. They were unable to conduct subgroup analyses for types and potency of cannabis use because of insufficient data, and most studies were conducted in the U.S., potentially limiting generalizability. Also, observational studies did not allow for causal inferences.

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