Heavy cannabis smoking is a major cause of gum disease, research suggests.
Gum disease is widespread in the adult population
An international team tracked the dental health of 1,000 people born in Dunedin, New Zealand, in 1972 and 1973.
They found heavy cannabis smoking was responsible for more than one-third of the new cases of gum disease among the group by the age of 32.
The Journal of the American Medical Association study follows work linking cannabis use to mental health problems, and lung disease.
Gum disease, one of the most common diseases of adulthood, is the second most common cause of tooth loss, after tooth decay.
It has long been linked to cigarette smoking, but this is the first study to look specifically at cannabis.
The researchers accepted that cannabis users also tended to smoke tobacco.
However, they said that even after allowing for this, heavy cannabis smokers had three times the risk of having established gum disease by the age of 32.
Even cannabis users who did not smoke tobacco developed gum disease.
Overall, 29% of the sample showed some signs of gum disease.
Lead researcher Professor Murray Thomson, of the University of Otago, said the effect was unlikely to be due to the direct impact of smoke on the gums as it was inhaled.
Instead, it was likely that toxins in the cannabis were absorbed into the bloodstream via the lungs, and then affected the body's ability to heal inflammation in the gums caused by the build up of bacteria on the teeth.
He said: "The problem is not the smoke itself - it's what is in the smoke.
"In the mouth, there is a fine balance between tissue destruction and tissue healing and the various toxins in the smoke disrupt that."
The fact that cannabis users tend to inhale more deeply than tobacco users might also exacerbate problems.
For the study heavy cannabis users were defined as those who used the drug for an average of at least 41 occasions a year between the ages of 18 and 32.
Susie Sanderson, of the British Dental Association, said: "It has long been recognised that smoking tobacco is a cause of gum disease.
"This study underlines the fact that gum disease can be caused not only by smoking tobacco, but also by smoking cannabis."
The research team also included scientists from King's College London, Duke University and the University of North Carolina.
Research published earlier this year found people who smoked one cannabis joint a day had a higher risk of lung cancer than those who smoked 20 cigarettes a day.
A separate study found that bullous disease - a form of emphysema - occurs 20 years earlier in cannabis smokers than in tobacco smokers.