Well, that is at least what the people with a financial stake in selling it tell us.
The reality is know very little about its long-term effects. This is mostly the result of it being near impossible to do long-term studies on it. All we have is case-control stuff without any useful dose response data.
10-20 years from now whe the first generation of perma-stoned coloradans gets into their 40s, we may be able to say whether the stuff is as innocuous as the Iindustry and their paid shills are telling us.
There are plenty of studies. The problem is finding impartial ones. Most of them have been funded by organizations with vested interests. Personally, I believe that to include government-funded studies, as well.
Donald Tashkin's 2006 and
2013 studies are probably the closest you're gonna get to impartial because Tashkin, who is a pulmonologist, admits to believing that marijuana is more dangerous than his own studies' results suggest. When a researcher admits to having issues with the results of his own study, that actually adds to the researcher's credibility, in my opinion.
I also doubt that legalization would have much effect prevalence of use. I live in a very small village, in a very small, very rural, very Republican county in New York. The village I live in has a population of < 600 and an average age of 48. We do not have a police department. The nearest police station is the Sheriff's office, which is about 29 miles away. This is the kind of place where no one locks their doors and everyone leaves their keys in their cars. It's like a cross between a Norman Rockwell painting and Mayberry, RFD.
And yet even here in the village that time forgot, no one -- NO ONE -- cares about marijuana laws. About half my friends and neighbors openly smoke it, and a few grow their own. I also know of at least half a dozen commercial growers within a 20- or 30-minute drive -- and I don't even use the stuff! I find it hard to imagine that legalization would change anything in view of how little attention is paid to the existing laws, even here in politically-conservative, law-abiding Mayberry.
I probably should mention that despite my strong support for legalization, I don't use marijuana. I tried it a few times in my youth and didn't care for it. It mainly just made me dizzy. I haven't used it since.
-Rich
EDIT: I've also overheard several very frank, very open discussions about the best lighting and pest control practices for marijuana cultivation at the local Tractor Supply store.