Quickest way to lose water weight in a day?
I gotta weigh in and need to know all I can do today to keep that weight off no matter how unhealthy it is.
Furosemide/lasix would probably be the strongest but you're only going to find that in a medicine cabinet if someone has some edematous problems or congestive heart failure. Most likely you'll find hydrochlorothiazide (HCTZ), which isn't as potent as a loop diuretic (such as furosemide) but should still be able to help you drop some water weight. Combine it will caffeine/theophylline etc and run around in a couple of layers of sweats for a long time. Salt restrict yourself, no salt. If you had more time I would suggest hydrating up with excess water and salt restricting so you get rid of all the sodium but that might take a couple of days.
You know how I said salt restrict yourself? Consider going NPO - no food or water until weigh in. Signs to look out for are light headedness, especially when going from laying/sitting to standing. This can be a sign of low BP due to excessive dehydration/hypovolemia.
Me again, another good option would be a laxative, not only will it dehydrate you, but it will force you to empty any remaining food from your GI. I don't advocate taking laxatives in combination with a prescription diuretic, that would probably cause excessive dehydration. But then again, I'm a doctor recommending you steal prescription pills from the elderly so what the hell.