Can diet soda cause cancer or kidney stones?!
You should wean yourself off of that high amount. Having that many will cause some problems for you in the long run, and it will put you at higher risk for kidney stones and cancer because you are pumping yourself full of unhealthy chemicals. If you feel that you can't live without it, then you are probably addicted (it can totally happen with soda!) and weaning yourself down to a normal, healthy amount will make you feel a lot better.
Diet soda used to "cause cancer in lab animals."
Of course, just about everything causes cancer nowadays.
Kidney stones can be caused by being dehydrated a lot. Caffeine is a diuretic (something that makes you pee).
So if you drink a lot of caffeinated beverages, it could lead to kidney stones.
I would go with the old standby: ANYTHING in too much quantity is bad for you. Six cans of soda per day isn't likely to kill you, but wouldn't six cans of sparkling apple cider or something healthier be better?
Diet soda is really bad for you because it uses artificial sugars. Most of the time it is aspartame which is also used in rat poison.
Not just diet soda, but caffeine itself is known to cause kidney stones.