New research suggests that teens using marijuana for medical reasons are more likely to become addicted to it.

Researchers at the University of Michigan found that adolescents with medical marijuana cards are 10 times more likely to say they are hooked on marijuana than youth who get marijuana illegally.

This finding doesn't mean that medical marijuana use is necessarily creating teen addiction to marijuana, especially considering how few teens hold medical marijuana cards. Researchers said it makes more sense that teens who feel dependent on the drug will seek marijuana cards to ensure a reliable, legal source.

For the study, 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.

They found that 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.

Carol Boyd, lead author of the study, 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," said Boyd, who is also a professor at the University of Michigan's School of Nursing. "More youth use medical marijuana that do not have a card than that have a card."

The findings are scheduled for publication in August in the Journal of Adolescent Health.