(KERO) — A new study casts some doubt on the use of medical marijuana for treating anxiety and depression.

The Harvard Medical study compared over 200 patients in two groups. One group got a medical marijuana card to treat their health symptoms immediately. The other had to wait 12 weeks.

It turned out that patients who got earlier access were 20-percent more likely to develop a marijuana use disorder. That means they became dependent on marijuana without showing improvement in their health symptoms.

Harvard researchers say their study shows patients on medical marijuana need closer supervision to have better outcomes.