And there's an actual toilet in Star Trek V in the brig, and maybe other references too. Actual smoking too in that film, not to mention in TNG, and elsewhere.Actually we do get some bathroom mentions on DS9. They called it "Waste Extraction" though for some reason only the Ferengi got to use that term. We also get a bathroom mention from Jake in "Explorers" were he is kind of confused as to how someone is supose to go on that ship. Also I think their might have been one on "Voyager." I think it was one of the episodes were the ship had almost no power and Nellix is talking to Janeway about how the Bolians bathroom issues might become a problem if they don't get the bathrooms working again.
Jason
But we're talking about TOS, by default, because this is the TOS forum. No one's ever spotted a toilet reference of any kind in TOS. With that thing that looks exactly like a stamped out cigarette butt I found in "A Piece of the Action," we have more direct evidence of smoking in TOS than toilet use, AFAIK, except of course that humans are known to need to use the bathroom, and the Enterprise has a lot of humans on board.