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(CNN) — Use of marijuana and other substances dropped in teenagers during the first year of the pandemic, according to a new study.

But adults' use of cannabis, illegal drugs and alcohol, including binge drinking, either stayed the same or increased compared to the two years before Covid-19.

"Substance use decreased between 2019 and 2020 among those aged 13 to 20 years," wrote first author Dr. Wilson Compton, deputy director of the National Institute on Drug Abuse.

However, "consistent declines were not seen in older persons other than tobacco use reductions, and cannabis use increased among adults ages 25 years and older," he and his coauthors wrote.

Teen use dropped

The study analyzed data from the Population Assessment of Tobacco and Health Study, which follows tobacco and other substance use over time among 49,000 US youths and adults.

"A particular strength of this study was the longitudinal design," said Joseph Palamar, an associate professor of population health at NYU Grossman School of Medicine, who was not involved in the study.

"This design allows us to look at changes among the same people over time as opposed to other national studies which compare different groups of people across time," he said.

Substance abuse dropped in teenagers between ages 13 and 17, according to the study published Tuesday in the journal JAMA Network Open.

Cannabis use among teenagers ages 13 and 15 dropped by 3.4 percentage points in 2020 compared to 2018 and 2019, while tobacco use declined by about 4 points, the study found. The use of other illegal or misused prescription drugs also fell 2.5 percentage points in this age group.

Use of marijuana in teens ages 16 and 17 dropped 7.3 percentage points in 2020 compared to 2018 and 2019. Tobacco use fell by over 10 points and misuse of drugs sank by nearly 3 percentage points. Binge drinking dipped by 1.6 percentage points across the age group.

"I think availability plays a big part," Palamar said. "If high schoolers are separated from their friends for a long time and stuck inside, they'll likely have decreased access to drugs.

"Even if a teen successfully obtained weed, this doesn't mean he or she had somewhere away from parents to smoke it if the whole family was on lockdown," he added.

Young adults drank more

The use of alcohol increased by over five percentage points (from 60.2 percent to 65.2 percent) among adults ages 21 to 24 years old in 2020 compared to the previous two years. Binge drinking, however, fell by 2.2 points.

Tobacco use fell by about 8 percentage points, but use of marijuana and other illegal or prescription drugs did not change significantly in this age group, according to the study.

Use of marijuana increased slightly in adults 25 and up, by 1.2 percentage points. Declines in other substance abuse in this age group were not significant, the study authors said.

Tobacco use fell for all adults, the study found. The number of young adults ages 18 to 20 smoking tobacco dropped by just over 15 percentage points in 2020 compared to 2018 and 2019. Smoking also declined by about 8 points in adults ages 21 and up over the same period.

Did the trend hold?

However, a drop in drug use during the early days of Covid did not mean the reduction continued as the pandemic wore on, said Palamar, who has been studying drug availability during that period.

"Decreases in use during the early months of Covid are meaningful, but we need to keep in mind that use of some drugs rebounded," Palamar said. "For example, we found that seizures of marijuana and methamphetamine decreased after the start of Covid, and then rebounded to a much higher rate later in the year."

A separate survey of people ages 19 to 30 found they were using marijuana and hallucinogens at high rates in 2021. The Monitoring the Future Study, published in 2022, found 11 percent of people in this age group used marijuana on a daily basis in 2021, while 43 percent said they had used it in the past year.

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