A new survey shows marijuana use by teens remains high, and officials say it will probably increase after Washington and Colorado decriminalized the drug last month.

"Based on what we know ... we are predicting that it's going to go up," says Nora Volkow, M.D., director of the National Institute on Drug Abuse. "Just the fact that there are some states that have made it legal ... will send a message" to teens throughout the country.

Already, the proportion of teens who consider marijuana to be harmful is the lowest it's been in decades, according to the 2012 "Monitoring the Future" survey of eighth, 10th, and 12th graders. The annual survey of teen drug use is conducted by researchers at the University of Michigan.

The survey's silver lining is that it shows the use of illicit drugs, alcohol, and cigarettes is declining.

Still, about 70% of eighth graders said they thought regular use of marijuana was harmful, while about 42% said they considered occasional use harmful. Those rates are the lowest since the survey began asking eighth graders that question in 1991.

Among 12th graders, the proportion who said regular use was harmful was about 44%, occasional use, about 21%. Those are the lowest rates since 1979 and 1983, respectively.

The survey shows that 6.5% of high school seniors said they smoke marijuana daily, which is about the same as last year but up from 5.1% five years ago.

Use of synthetic marijuana, known as K-2 or Spice, was stable in 2012, with slightly more than 11% of high school seniors reporting they had used it in the past year, the survey shows.

Read more from the National Institute on Drug Abuse and WebMD.