The study was led by W. Murray Thomson of the school of dentistry at The University of Otago, New Zealand, who measured gum recession at three sites on each tooth at ages 26 and again at 32. The study subjects are part of a longitudinal health and development study that has been tracking nearly 1,000 people born in Dunedin, New Zealand, in 1972 and 1973.

The link between gum disease and marijuana use emerged from a statistical analysis that controlled for gender, dental care, socio - economic status and how clean or dirty the teeth were. Most of the self-identified heavy marijuana users also were tobacco-users, but that factor was controlled statistically. The researchers also were able to focus on study participants who were not tobacco-users, and they still found a link between marijuana use and gum disease.

The precise physiology of smoke's effect on the gums is still not understood, but the team believes it interferes with immune function, inflammatory response and peripheral blood flow in the gums.

In addition to Moffit and study Thomson. the included Richie Poulton. David Welch and Robert Hancox of the Dunedin School of Medicine; Jonathan Broadbent of Otago's Dental School; De Kapel Hill; van Beck of the Dental School at the University of North Carolina van James at and Avshalom Caspi of Duke University. Support came from the U.S. National Institutes of Health, the UK Medical Research Council and the Health Research Council of New Zealand.