Specifically if you never go back to regular gravity. Ex: you go to live on another planet with say 3/4 Earth’s gravity and never return to Earth.
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The short answer is we don’t know.
The other answers here are mostly talking about microgravity, which is what most people would call “zero gravity” (there are technical reasons for the different name that aren’t relevant here). I.e. floating around, not particularly affected by gravity at all. That’s what they have on the ISS, and there are people who have stayed there for many months at a time, so that gives us decent data on that. Though even then we still don’t know what it’d be like to live there permanently.
We have next to no data about the health effects of lower gravity (that is, still noticeable gravity, but less than 1 g, like you’d have on the Moon or on Mars). We just don’t have a good way to test it. The longest Apollo missions only stayed on the Moon for a few days, and that just wasn’t enough to test it.
We can’t just assume that the health affects would be the same as what you get in microgravity. Lunar gravity and microgravity are not the same thing and there’s no reason to assume the health impacts would be the same.
It’s generally assumed that really low gravity isn’t healthy for long periods, but we currently have no way of knowing where the threshold is and for how long you could stay. Maybe you could be fine in conditions as low as 0.5g, maybe anything below 0.9g is dangerous in the long term. We can’t tell.
This is actually one of the motivations for setting up a Lunar colony–it would give us a way to test this in a way that’s not as high risk as a Mars mission. On a Mars mission, if you found out some serious health issue that would require an astronaut to return to Earth, they’re months away from getting back. On the Moon, it’ll only be a few days before they can get back to 1g. Much less risky.
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