It's relatively safe. I've chewed up 3,000mg of Tylenol at a time, though i always am afraid when i do it. It starts causing problems when you're chewing HANDFULS of Tylenol on a day to day basis. I've never *felt* any kidney/liver pains, but i'm sure that i've done some damage in the 12 or so years i've been popping pills.
If you start taking double that dose or more on a consistent basis, look up "Cold Water Extraction" or CWE threads on here and you'll find a way to get rid of almost all the Tylenol in your pills.