Drano (at least the original formula) can damage plastic pipes, which is why they don't recommend too many attempts. It also gives off some hydrogen as it works, but that quickly dissipates.

If you've got good metal pipes, reusing Drano every hour or so isn't an issue. On the other hand, if two or three uses doesn't help, then its likely that the clog is far enough down that it simply isn't reaching where it needs to get to, to work.