Marijuana smokers can develop gum disease even if they don't also smoke cigarettes, according to new research.

A study of 900 adults ages 26 to 32 in New Zealand found that one-third of the new cases of gum disease discovered among the group were due to smoking marijuana. "That really surprised us," lead researcher Murray Thomson said.

The researchers said that even those who only smoked marijuana once a week increased their risk of gum disease threefold compared to nonsmokers. Toxins in marijuana impair circulation and prevent the gums from healing themselves, Thomson and colleagues said.

The study was published in the Feb. 6, 2008 issue of the Journal of the American Medical Association.