Soda *that* flat and unappealing may well of suffered a containment leak in the bottle. (It's rare to have it go completely dead under normal circumstances, unless the bottle is literally *months* old.) I definately wouldn't use it for anything involving passing it through your gastrointestinal tract.

On the bright side diet soda is so fundimentally lacking in nutrative value nothing will grow in it, so you're probably safe if you insist. ;^>