The study, published in The Lancet Psychiatry, found that negative mental health outcomes were disproportionately reported by girls and older adolescents (13-18-year-olds), compared to same-age peers before the pandemic.
At the same time, it revealed a decline in cigarette smoking, e-cigarette usage and alcohol intoxication among 15-18-year-old adolescents during the pandemic.
"The decrease observed in substance use during the pandemic may be an unintended benefit of the isolation that so many adolescents have endured during quarantine," said researcher John Allegrante from the Columbia University in the US.
Previous studies of adolescents during Covid-19 found evidence of increased mental health problems and certain types of substance use that had been rising before the pandemic.
This study, however, compares current data with several pre-pandemic time points, which enabled the researchers to separate the effect of Covid-19 from other recent, downward trends in adolescent mental health.
According to the researchers, prior studies have not been designed to determine whether clinically relevant levels of depression -- as opposed to self-reported depressive symptoms -- and substance use have increased during the pandemic.
The study "differs in methodology from previous studies in that it tracked the population-based prevalence of mental health outcomes and substance use over several years to better understand the potential effects of Covid-19 from recent upward trends in adolescent mental health problems, the team said.