Well, both Tylenol and coke are processed by the liver so you'll just be putting more exertion on it.
No, it probably won't
kill you, unless you've done a whoooooole lot of coke but it may make your coke comedown that much shittier. You're not supposed to take Tylenol after a night of drinking because it makes your liver work even harder to process the alcohol and I'd assume it would be very similar with blow.
If anything I'd take just Diphenhydramine (the part of Tylenol PM that makes you sleep) without the Tylenol.
The best cure for any comedown is of course good ol' fashioned bud though.
