Chicago ? Though fewer eighth-, 10th- and 12th-graders across the country report illicit drug use, those who do are increasingly turning to prescription drugs, according to a University of Michigan study released Tuesday at the White House.

The use of OxyContin, first measured by the researchers in 2002, was slightly higher this year for all three grades. At least one in every 20 high school seniors has tried the narcotic in the past year, researchers found.

Wilson Compton, division director at the National Institute on Drug Abuse, said he was troubled by the finding.

“Prescription drugs remain at high and very concerning levels,” he said. “We need to do a better job of communicating the risks of these prescription drugs and protecting youth from what can be dangerous in the long run.”

The study also offered reasons for optimism.

The proportion of eighth-graders reporting use of an illicit drug at least once in the 12 months before the survey dropped nearly by half, from 24 percent in 1996 to 13 percent in 2007.

The decline was less dramatic for 10th-graders, from 39 percent in 1997 to 28 percent. Use declined among 12th-graders from 42 to 36 percent in the same period.

The use of alcohol by teens, like their use of many of the illicit drugs, has declined since the mid-1990s. Smoking rates continued a gradual decline in grades eight and 10 in 2007.

Among those drugs apparently used less by teens are marijuana and stimulant drugs like amphetamines. Cocaine was the one stimulant that did not show a decline in usage this year. Though its use peaked in the late 1990s, then declined for a year or two, it has held relatively level in recent years. Between 2 and 5 percent of students in each of the three grades surveyed reported using cocaine during the previous year.

MDMA, or Ecstasy, showed signs of increased use. Though the popularity of the party drug plummeted in the early 2000s, use has begun to increase again in the upper grades.

“There is evidence here of this drug beginning to make a comeback,” wrote Lloyd Johnston, a University of Michigan researcher and principal investigator of the study.

The study also looked at the use of over-the-counter cough and cold medications taken to get high. The cough suppressant dextromethorphan is an active ingredient in most.

Usage rates, which ranged from 4 percent in eighth grade to 7 percent in 12th grade, have remained fairly steady during the last decade, with a slight decrease this year in grade 12.

“There is little evidence yet of much improvement,” Johnston wrote.

The study, “Monitoring the Future,” is in its 33rd year and funded by the National Institute on Drug Abuse. The survey sampled 48,025 students from 403 secondary schools.