There is no guarantee that you will suffer harm from a single drink. What it really means is that any amount of alcohol consumption carries some (possibly minute) risk of harm. This is not, IMO, equivalent to "unsafe", which generally means something well outside the bounds of normal risks that most people already take on in their everyday lives.
If we accepted that "some risk of harm" = unsafe, we would have to describe using the stairs as unsafe, taking a shower as unsafe, putting up Christmas lights as unsafe, etc.