there is really no way of proving that smoking out of a can causes the things that a lot of people claim, but logic tells me that poking a few holes in an aluminum can, and then holding a flame near aluminum, the dye on the can, and the outer and inner coatings can't be good for you, plus instead of tasting like weed, it tastes like a trifecta of weed, whatever beverage was in that can, and shit. There is always a better alternative at hand, just be creative. I pretty much always have a pack of zig zags in my wallet, but if you can't roll for shit just use an apple or something.
sparknotes: smoking out of a can is better than not smoking at all, but you really should be able to find something else to smoke out of.