Using marijuana or drinking alcohol in middle and high school may negatively impact mental health and academic outcomes.
The study, led by Elizabeth D'Amico, senior behavioral scientist at the RAND Corporation - an American nonprofit global policy organization - was recently published in the journal Addiction.
Alcohol and marijuana use are common among adolescents in the United States.
Approximately 8.7 million youths aged 12-20 report having consumed alcohol in the past month, while 6.5 percent of 8th-grade students and 14.8 percent of 10th-graders in the U.S. currently use marijuana.
For their study, D'Amico and colleagues set out to investigate how alcohol consumption and marijuana use in middle and high school might influence academic outcomes.
Youth 'need to better understand the harms of marijuana use'
The research involved 6,509 youths from 16 middle schools across Southern California - all of whom were part of an alcohol and drug use prevention program called CHOICE.
Between the ages of 11 ½ and 17 years - spanning from middle to high school - the students completed a total of seven surveys, in which they were asked about their use of alcohol and marijuana.
The final survey they completed also asked about their academic performance, as well as social functioning, delinquent behavior, and mental and physical health.
Compared with youths who engaged in lower or no alcohol and marijuana use during middle and high school, those with greater use were found to have poorer academic preparedness and were more likely to have delinquent behavior.
Furthermore, youths who engaged in greater marijuana use alone during middle and high school were found to have poorer academic performance and poorer mental health.
"Many youth tend to think that alcohol use has more consequences than marijuana use and therefore view marijuana use as safer than drinking.
However, youth need to better understand the harms of marijuana use, such as the potential effect on their developing brain and how it can affect performance in both adolescence and adulthood."
Risks of alcohol, marijuana use greater for non-white youths
White youths were more likely to engage in alcohol and marijuana use than non-white youths, according to the team.
However, the researchers found that Hispanic and multi-ethnic youths were at greater risk of poorer academic performance as a result of higher marijuana and alcohol use, compared with white youths.
Additionally, Asian, black, and Hispanic youths were found to be at greater risk of poorer academic preparedness than white youths, while Asian and multi-ethnic youths had poorer physical health.
Dr. D'Amico says their findings highlight the need to address alcohol and marijuana use early on, particularly for non-white adolescents.
"One approach may be to increase protective factors such as parental support or the adolescent's ability to resist temptations to use these substances," she adds.
The researchers note that there are some limitations to their study. For example, they did not account for racial discrimination, parental involvement, or neighborhood quality - factors that may have influenced alcohol and marijuana use or academic and mental functioning during the study period.