LOS ANGELES (Reuters) - The use of alcohol, marijuana, prescription medications and illicit substances declined among U.S. teens again in 2016, continuing a long-term trend, according to a study released on Tuesday by the National Institutes of Health.

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But the research found that high school seniors were still using cannabis at nearly the same levels as in 2015, with 22.5 percent saying that had smoked or ingested the drug at least once within the past month and 6 percent reporting daily use.

“Clearly our public health prevention efforts, as well as policy changes to reduce availability, are working to reduce teen drug use, especially among eighth graders,” Dr. Nora Volkow, director of the National Institute on Drug Abuse, said in a statement accompanying the study results.

“However, when 6 percent of high school seniors are using marijuana daily, and new synthetics are continually flooding the illegal marketplace, we cannot be complacent,” Volkow said.

The annual survey, part of a series called Monitoring the Future which has tracked drug, alcohol and tobacco use among teens since 1975, also found that during 2016 there was a higher use of pot among 12th graders in states with medical marijuana laws.

According to the study, marijuana and e-cigarettes are more popular among teens than regular tobacco, with a large drop in the use of tobacco cigarettes among 8th, 10th and 12th graders.

In 2016, 1.8 percent of high school seniors smoked half a pack or more of tobacco cigarettes per day, compared with 10.7 percent in 1991.

The use of alcohol has seen similar declines, according to the research, with 37.3 percent of 12th graders reporting this year that they had been drunk at least once, down from a peak of 53.2 percent in 2001.

The analysis found that the use of illicit drugs other than marijuana by teens was at its lowest levels since tracking began.

The study, which is sponsored by the National Institute on Drug Abuse and conducted by researchers at the University of Michigan at Ann Arbor, surveyed 45,473 students from 372 public and private schools.

(This version of the story corrects final paragraph to show study was funded by the National Institute on Drug Abuse.)