Dec 15, 2016 09:07 AM EST By Julio Cachila, UniversityHerald Reporter

An annual survey has found that teen usage rates of marijuana, and other drugs, have been steadily declining, with some rates at their lowest levels in decades.

In the annual Monitoring the Future survey, the National Institute on Drug Abuse (NIDA), a part of the NIH, asked 8th-, 10th-, and 12th-graders about their usage of marijuana and they found that the usage rates of these students declined or remained the same as last year's, reports U.S. News. In fact, some of these students are finding it harder to obtain the drug even with the current permissions that several states have for it.

This year, only 34.6% of 8th-graders say it is easy to obtain marijuana, down by 2.4 percentage points from last year's 37%. Marijuana use among them dropped significantly, from 6.5% in 2015 to 5.4% this year.

Among 10th-graders, only 64% said it can easily be obtained. Although this isn't considered a significant drop from last year, it is the lowest rate ever in two decades. Reported use kept declining over a five-year period.

High school seniors, on the other hand, report greater accessibility with 81% saying it is easy to acquire. This is slightly higher than last year's rate which is considered the lowest rate yet. Usage rates appeared stagnant among students in the 12th grade.

Students in states with medical marijuana laws reported higher usage rates compared to students in non-medical marijuana states, at 38.3% versus 33.3%.

The findings, which are gathered from asking students every year, surprised the researchers.

"We had predicted based on the changes in legalization, culture in the U.S. as well as decreasing perceptions among teenagers that marijuana was harmful that [accessibility and use] would go up. But it hasn't gone up," Dr. Nora Volkow, director NIDA said.

"I don't have an explanation. This is somewhat surprising."

Lloyd D. Johnston, Ph.D., principal investigator at the University of Michigan's Institute for Social Research, said the decline in teen usage is good news.

"The declining use of many drugs by youth is certainly encouraging and important," Dr. Johnston said in a press release. "But we need to remember that future cohorts of young people entering adolescence also will need to know why using drugs is not a smart choice. Otherwise we risk having another resurgence of use as was seen in the 90s."

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