(CNN) - Most teens may be "Above the Influence" when it comes to cocaine and cigarettes, but marijuana use is growing among students.
Sixty percent of U.S. high school seniors do not see regular marijuana use as harmful to their health, according to this year's Monitoring the Future survey from the National Institute on Drug Abuse. More than a third of the seniors surveyed reported smoking marijuana in the past 12 months.
Each year, the Monitoring the Future survey asks eighth-, 10th- and 12th-graders about their drug and alcohol use and their attitudes toward illegal substances. For 2013, more than 41,000 students from 389 U.S. public and private schools participated.
Only 2.4% of high school seniors reported using marijuana daily in 1993; this year that percentage nearly tripled -- to 6.5 %. And it's not just the older students -- more than 12% of the eighth-graders surveyed said they had used marijuana.
"It is important to remember that over the past two decades, levels of THC -- the main psychoactive ingredient in marijuana -- have gone up a great deal," said Dr. Nora Volkow, director of the National Institute on Drug Abuse, in a statement. "Daily use today can have stronger effects on a developing teen brain than it did 10 or 20 years ago. ... The children whose experimentation leads to regular use are setting themselves up for declines in IQ and diminished ability for success in life."
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Teens also continue to abuse prescription medications such as Adderall, which is commonly used to treat attention-deficit hyperactivity disorder, and Vicodin. But while alcohol use is still high -- close to 40% of seniors reported drinking in the past month -- it's been on a steady decline since its peak in 1997.
There's more good news: Use of K2 or Spice, also called synthetic marijuana, dropped 3.4 percentage points among high school seniors, and less than 1% of students in all three grades reported using bath salts, a family of drugs that contain synthetic chemicals.
"Synthetic drugs are particularly dangerous because their ingredients are unknown, they have not been tested for safety and their ever-changing ingredients can be unusually powerful," said lead researcher Lloyd Johnston. "Users really don't know what they are getting."
Use of inhalants, cocaine and heroin are also on the decline. Four percent of seniors reported using Ecstasy, or MDMA, which is "considerably lower" than in 2001, when 9.2% reported using the synthetic drug, the report authors said.
Survey results for the teens' tobacco use seem to reflect the decline seen in adults. For the first time, less than 10% of the students in all three grades reported smoking cigarettes in the past month. But teens aren't giving up nicotine just yet. More than 21% of seniors acknowledged smoking tobacco with a hookah this year, compared with 18.3% in 2012.
The survey results must be considered cautiously as the participants may have been afraid to report their true use of illegal substances, or may have put down false information.
The team of researchers who led the study will be exploring the link between state laws and students' marijuana use going forward, the report authors said. Many of the teens who used marijuana in states with medical marijuana laws said they obtained the drugs through their own or someone else's prescription.