The biggest risks are that single-use bottles are usually pretty difficult to clean (usually a narrow opening). The second biggest, which is related, is that those single-use bottles usually aren't very rigid and will tend to make small cracks in the surface as the material flexes which makes things even harder to clean. After that, all the cracks that will develop will mean it'll leach out the bad stuff in the plastics far faster than if you had some other kind of water bottle.

If you just opened it and drank the drink in it, there's probably no harm in filling it soon after and using it a few times like that. Using that same disposable bottle for a few months is probably not a good idea.