You must already know
that it is
not harmful. You know
that it is done by people, and therefore must
not be harmful. So the fact that you ask shows that you don't want to do it and are looking for some medical reason that you can use to tell him you shouldn't be swallowing it.
But that isn't the case. It's harmless. BUT, you don't need a medical reason to back you up. If you don't like it, if it makes you gag, if you find it unpleasant.....don't do it.