Fewer U.S. adolescents are participating in programs designed to prevent substance abuse, a study suggests.

Participation in these programs has dropped significantly since the early 2000s, researchers found.

While 48% of adolescents attended prevention programs in 2002-2003, only 40% did so in 2015-2016, the researchers report in the Journal of Adolescent Health.

That’s a 16.5% decline, Christopher Salas-Wright of the Boston University School of Social Work and his colleagues note.

In particular, participation dropped among Latino youth and youth from rural areas and economically disadvantaged backgrounds.

The findings are drawn from the long-standing National Survey on Drug Use and Health. An interviewer typically brings a laptop to participants’ homes, so they can complete the survey privately.

With federal funding for substance abuse prevention nearly halved between 2005 and 2015, Salas-Wright told Reuters Health, it “certainly seems intuitive” that funding cuts were partly responsible for the drop in participation. However, the study did not specifically examine this question.

The Trump Administration last year suspended the National Registry of Evidence-Based Programs and Practices, a searchable database that helped people find intervention methods to prevent and treat mental illness and substance use disorders.

Until the suspension, “states were able to pick from an extensive list of approved model programs,” study co-author Flavio Marsiglia said in an email.

“Policymakers at different levels of government may have switched resources away from universal prevention in schools, in part to respond to the opioid epidemic,” Marsiglia said.

He added that while the U.S. opioid crisis is a matter of great concern, alcohol and marijuana use are more common among adolescents, who require help resisting addiction to these substances.

Although significant declines for school-based participation were seen across demographics, the data don’t show whether adolescents were not attending existing programs or because the number of available programs had dropped.

Salas-Wright also points out that rates of adolescent substance use and risky behavior have declined in recent years.

“It may be that rates in prevention programming are, in part, down because fewer young people are using alcohol and drugs, getting into trouble, and then are mandated to go to a program designed to prevent future use,” he told Reuters Health.

Linda Richter, Director of Policy Research and Analysis at New York-based nonprofit Center on Addiction, who was not involved in the study, said the findings appeared consistent with her experience, but she flagged a limitation.

In the survey, “prevention program participation in schools was measured by a single item asking young people if they’ve had a special class about drugs or alcohol in school,” she said. “Our experience with school-based prevention is that this question might be too narrow . . . for example, many schools hold assemblies where they bring in speakers to discuss substance use prevention with the student body.”

While researchers advocate “evidence-based prevention” as a cost-effective solution to substance abuse problems, Richter notes that demonstrating the effectiveness of programs is very difficult.

“The outcomes are often seen years after the implementation of the program, and research rarely is funded to cover longer-time results,” she said.