A new University of Michigan study finds that teens using marijuana for medical reasons are 10 times more likely to say they are hooked on marijuana than youth who get marijuana illegally.

The study is the first to report on a nationally representative sample of 4,394 high school seniors and their legal or illegal medical marijuana use as it relates to other drug use. In the study, 48 teens had medical marijuana cards, but 266 teens used medical marijuana without a card.

Carol Boyd, the study's lead author and professor at the U-M School of Nursing, said she doesn't believe that medical marijuana use is necessarily creating teen addiction to marijuana, especially considering how few teens hold medical marijuana cards. She said it makes more sense that teens who feel dependent on the drug will seek marijuana cards to ensure a reliable, legal source.

Researchers looked at three types of marijuana users: medical users; those who used another's medical marijuana; and those who acquired marijuana from nonmedical sources such as street dealers. For each group researchers analyzed five risk behaviors connected to marijuana and other drug use.

Teens who used other's medical marijuana were at highest risk for engaging in all five risky behaviors, including using marijuana more frequently to get high and using alcohol and prescription pills. However, they were just four times as likely to say they're hooked on marijuana, as opposed to 10 times as likely for medical marijuana users.

Users who got marijuana from nonmedical sources, such as street dealers, were the largest group, but they had the lowest likelihood of engaging in the risky behaviors.

Boyd said the data illuminate shortcomings in medical marijuana policy.

"I think that medical marijuana laws are failed policy and that these data lend support to my position," Boyd said. "More youth use medical marijuana that don't have a card than that have a card."

According to the U-M C.S. Mott Children's Hospital National Poll on Children's Health, 4 in 5 people say adults should not be allowed to use medical marijuana in front of children.

The study, "Adolescents' use of Medical Marijuana: A Secondary Analysis of Monitoring the Future Data," is online now and is scheduled for publication in August in the Journal of Adolescent Health. The study used data from the 2012 and 2013 Monitoring the Future study, an ongoing study conducted at U-M's Institute for Social Research.

Boyd also has appointments at the U-M Addiction Research Center and the Institute for Research on Women and Gender.