The report showed no significant changes from 2004 in use of several drugs, including marijuana, sedatives and prescription stimulants. Meanwhile, teens' use of the prescription narcotic painkiller OxyContin continued a years-long rise, researchers said.
Overall, 15.8 percent of teens say they've used any illegal drug in the past month, down slightly from 2004. Use among high school seniors rose by 1 percent; it dropped 1 percent among sophomores and stayed unchanged among eighth graders.
"Most declines halted this year," said Lloyd Johnston, a University of Michigan researcher who leads the study, known as the Monitoring the Future Survey. The annual survey asks nearly 50,000 U.S. students in 400 schools about their illicit drug use.
Despite the flat 2005 figures, Bush administration officials focused on clear reductions in illegal drug use among U.S. teens over the longer term. Overall, students' use of most drugs -- including marijuana, anabolic steroids, cigarettes and alcohol -- have dropped 19 percent since 2001.
Approximately 700,000 fewer American students are using drugs now than at the beginning of the decade, said John Walters, director of the White House Office of National Drug Control Policy.
"We have a broad set of behaviors by young people that are going in a healthy direction," he told reporters. "Across the board we're having sustained declines."
The study also showed a significant 1.4 percent drop in methamphetamine use among seniors, with a smaller decline among sophomores over the past year. Nearly 9 percent of 12th graders reported using the drug -- also known as "speed" or "meth" -- within the past year.
Officials said they remained concerned about several trends in the report, including a continuing rise in OxyContin abuse among high school seniors and increasingly common use of widely available but highly toxic inhalants.
They also warned that significant drops in teen smoking could be coming to an end. Overall, smoking rates have dropped between one-third and one-half for U.S. teens since 1997, the year tobacco companies settled a massive lawsuit by states by agreeing to curtail advertising that might attract children and to pay for anti-smoking campaigns.
But smoking among eighth graders has stayed flat at roughly 5 percent since 2002. At the same time, eighth graders' disapproval of cigarettes and the risk they see in smoking are both down.
Johnston called the trend "worrisome" and cautioned that declines in smoking are likely to stop as today's eighth graders move through high school.
"It appears to me they will end in the next couple of years in the upper grades," he says.
Johnston blamed waning funds for youth anti-smoking campaigns for teens' apparent shifting attitudes toward smoking. Cigarettes remain the leading preventable cause of death in the United States and are blamed for an estimated 440,000 early fatalities per year.
Tobacco company funding for youth anti-smoking marketing under the 1997 settlement has dropped from $308 million in 2001 to a scheduled $40 million in 2006, said Cheryl Healton, president of the American Legacy Foundation, the group administering the campaign. The campaign includes widespread "truth" anti-smoking ads aimed at teens.