(Reuters Health) - High school students in the U.S. are a lot more sober than they used to be, a new study suggests.

The proportion of high school students in their final year who have never tried alcohol or drugs increased fivefold from 1975 to 2014, and surged nearly as much for younger students between 1991 and 2014, the study found.

"The message that substance use is unhealthy seems to be gaining in popularity and 'sticking' with kids," said lead study author Dr. Sharon Levy, director of the Adolescent Substance Use and Addiction Program at Boston Children's Hospital.

For the study, researchers examined four decades of data from a national survey asking 8th-, 10th- and 12th-grade students about their use of cigarettes, alcohol, marijuana and other substances. Questions touched on lifetime use as well as use in the past 30 days.

In 1976, only 5 percent of 12th graders said they abstained from all substance use, meaning 95 percent of them had at least sampled alcohol, tobacco, marijuana or other drugs. By 2014, the proportion of these high school seniors abstaining from substance use had climbed to 26 percent.

A similar trend was seen for 10th graders.

The largest change was seen among 8th graders, with abstinence rates climbing from 24 percent in 1991 to 62 percent in 2014.

Among students who did report at least some use of alcohol, tobacco, marijuana or other substances, the proportion who reported abstaining in the past 30 days also rose during the study period.

For high school seniors in particular, prior-month abstinence rates dipped from 23 percent in 1976 to 16 percent in the early 80s, then climbed to about 40 percent by the end of the study period.

Patterns were similar for younger students, with higher rates of abstinence overall than among the older teens.

Drinking rates steadily dropped during the study, with the proportion of 12th graders never trying alcohol rising from 8 percent in 1976 to 34 percent in 2014.

With cigarettes, 25 percent of the high school seniors abstained in 1976 compared with 66 percent in 2014.

Marijuana abstinence, however, peaked at 67 percent in 1992 and then dipped to 56 percent by the end of the study.

Laws restricting sales of tobacco and alcohol to young people and shifting cultural attitudes about smoking and drinking have helped drive up abstinence rates, Levy said by email.

But legalization of marijuana in many U.S. states and the surging popularity of e-cigarettes among young people both threaten the encouraging trends seen in the study, she said.

"Both e-cigarettes and marijuana are typically promoted as safe, harmless, or, in the case of marijuana, even medication, and these memes have the potential to override the broader message that substance use is unhealthy," Levy said.

One limitation of the study is that surveys were administered in school, and teens who miss school or drop out may be more likely to smoke and drink, the researchers note in Pediatrics. This may mean the study overestimated abstinence rates.

Even so, the results add to evidence demonstrating the success of public health efforts to curb teen smoking and drinking, said Brendan Saloner, a researcher at the Johns Hopkins Bloomberg School of Public Health in Baltimore who wasn't involved in the study. Price hikes and taxes have been a deterrent along with age restrictions on purchases.

"A generation ago it was much easier for youth to get access to cigarettes than it is today," Saloner said by email.

Vaping and smoking pot, however, are a growing concern, said Dr. Scott Hadland, a pediatrics researcher at Boston University School of Medicine who wasn't involved in the study.

Compared to tobacco and alcohol, "marijuana and e-cigarette use have not declined as consistently, and binge drinking remains common," Hadland said by email.

"Drug overdose deaths in 2015 were more than double their level in 1999," Hadland added. "Motor vehicle crash deaths remain the single leading cause of death among teens, and many of these involve alcohol or other substances."

SOURCE: bit.ly/2NZx2i9 Pediatrics, online July 19, 2018.