A longitudinal study which tracked 1,037 New Zealanders from birth to middle age found marijuana use did not cause physical problems, with one notable exception: periodontal health. The biggest issue with smoking weed, it seems, is you’ll get bad gums later in life.

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The researchers at Arizona State University assessed over a dozen physical health parameters like lung function, systemic inflammation, waist circumference, HDL cholesterol, triglycerides, blood pressure, glucose control and body mass index. Participants were asked to self-report their marijuana use at ages 18, 21, 26, 32 and 38.

Results suggest those who used marijuana over the last 20 years showed an increase of periodontal disease compared to those who didn’t, starting from age 26. Marijuana smokers did not differ from non-users on any other health measure.

“We don’t want people to think, ‘Hey, marijuana can’t hurt me,’ because other studies on this same sample of New Zealanders have shown that marijuana use is associated with increased risk of psychotic illness, IQ decline and downward socioeconomic mobility,” said Madeline Meier, an assistant professor of psychology at Arizona State University

“What we’re seeing is that cannabis may be harmful in some respects, but possibly not in every way,” said study co-author Avshalom Caspi, the Edward M. Arnett Professor of psychology and neuroscience at Duke. “We need to recognize that heavy recreational cannabis use does have some adverse consequences, but overall damage to physical health is not apparent in this study.”

One might argue that it’s not smoking pot per se that causes bad gums, but surrounding factors like mixing marijuana with tobacco or the fact that marijuana users are less likely to brush and floss. The study confirmed this, but even after controlling for tobacco use, dental hygiene, and alcohol consumption, the association between marijuana use and decrease in periodontal health remained.

“Physicians should certainly explain to their patients that long-term marijuana use can put them at risk for losing some teeth,” said Terrie Moffitt, the Nannerl O. Keohane University Professor of psychology and neuroscience at Duke and co-director of the Dunedin Multidisciplinary Health and Development Study, from which these data were gathered.

As Professor Meier mentioned, it’s important to remember that this study focused solely on physical health. Regular marijuana use is bad for teens’ brain, while heavy use can damage pleasure centers in the brain and affect memory in adults.