Seem to recall that in one of the earlier Duke Nukems, you could piss in a urinal or something to that effect. That seemed like ages ago, so perhaps it was just flushing a toilet and he made some sarcastic comment.

You won't drown, asphyxiate, or anything like that.

All a health potion does is restore the numbers back to your body, and there's nothing biological about it. Took a big abdominal wound and 14 numbers fell out of your body through your stomach? The potion puts 12 numbers back into you so now you just have a periodic ulcer or bad gas. As we've learned form countless RPGs, sleeping will cure the remaining minor stomach malady and restore those last two numbers.