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There’s been a recognition that cigarette smoking can have a major effect on bone health for decades––increasing the lifetime risk of hip fracture by about half. It also appears to impair bone healing––so much so that surgeons ask if they should be discriminating against smokers, because the bone and wound healing complication rates are so high. But what about smoking marijuana?

There is accumulating evidence to suggest that cannabis compounds play important roles regulating bone mass and bone loss. Yeah, but are they friend or foe? “Results from research on [cannabis compounds] and bone mineral density in rodent models have been inconsistent. Some studies show increased bone formation, others [show increased] bone loss, and…others [show] no association” at all. This variation in results may be due in part to differences in the mouse strain used. But if you can’t even extrapolate from one mouse to another, how can you extrapolate to human beings?

So, what if you just measure cannabis use and bone mineral density in people? Thousands of adults were tested and asked about their cannabis use, and there did not appear to be any link between the two; that’s a relief. Though in this study, “heavy” cannabis use was defined as just five days or more of use in the last month. They didn’t ask beyond that. So, theoretically, someone who’s just smoked five joints in their entire life could be categorized as a “heavy user.”

How about cannabis use on five thousand separate occasions over a lifetime? Now that’s a heavy user, decades of regular use. And in that case, heavy use was associated with both low bone mineral density and an increased risk of bone fractures. About double the fracture rate––presumably due to lower bone density in the hip and spine, though heavy cannabis users were also skinnier on average, and skinnier people have lighter bones.

Hip fracture risk goes down as your weight goes up; nearly half of underweight women have osteoporosis. But less than 1% of obese women do, which makes total sense. Being obese forces your body to make your bones stronger to carry around all that extra weight. That’s why weight-bearing exercise is so important to constantly put stress on your skeleton: it’s use it or lose it. That’s why astronauts can lose a percent of their bone mass every month. Their bodies aren’t stupid; why waste all that energy making a strong skeleton if you’re not going to put any weight on it?

So, maybe that’s the only reason heavy cannabis users have frailer bones––is because they tend to be about 15 pounds lighter? Wait a second, users are slimmer? What about the munchies? The lower BMI of heavy cannabis users may seem counterintuitive, given the appetite stimulation, but this isn’t the first time this has been noted.

Pop culture “depicts marijuana users as a sluggish, lethargic,…unproductive subculture of compulsive snackers.” And it’s true that marijuana has been found to increase food intake; a single hit can increase appetite. And so, you’d expect obesity rates to rise in states that legalized it. But, if anything, the rise in obesity appeared to slow after medical marijuana laws were passed, whereas it appeared to just keep rising in other states.

The reason pot smokers may be slimmer is because of the effect of smoked marijuana on metabolism. We’ve known for nearly 40 years that within 15 minutes of lighting up, your metabolic rate goes up by about 25%, and stays there for at least an hour. So, that may be playing a role.

So is that why heavy cannabis use is associated with lower bone mineral density and increased risk of fractures? They’re just not as overweight? No. Even taking BMI into account, heavy cannabis use appears to be an independent predictor of weaker bones.

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