A couple of posts in this thread have mentioned the inherent risks in high dose and/or long term acetaminophen (or medications containing acetaminophen) therapies, and rightly so. It's rough on the liver, and anyone who takes a lot of acetaminophen should probably have a discussion with their doctor(s) about prudent periodic liver function blood tests.

Unfortunately, some folks like smae (and myself) don't have a lot of choice. I don't tolerate NSAIDs (Non-Steroid Anti-Inflammatory Drugs) well.

In my own case, I include NAC (N-Acetyl Cisteine/N-Acetylcysteine/N-acetyl-L-cysteine) in my daily supplement regime. NAC is an OTC supplement which, among other things, is administered intravenously as the first-line antidote for Tylenol overdose.

For more information:
Wiki: Acetylcysteine
Google: NAC tylenol

N.B. There is no proof/study that I'm aware of at this time that suggests that NAC taken prophylactically will prevent/reduce liver damage from acetaminophen. OTOH, taken as directed, and with knowledge/consent of one's doctor, it shouldn't be harmful either - It couldn't hoit! NAC has some other potential benefits as well.

There is an archived Usenet newsgroup discussion (sci.med,sci.med.pharmacy) from 1995 on this subject between two physicians (Steve Harris, M.D. & Jonathan R. Fox, M.D.)
http://yarchive.net/med/tylenol.html

Reviewing the above Google search, there may also be some similar potential prophylactic benefit from the supplement S-Adenosyl methionine (SAM, SAMe, SAM-e) but I'm not personally familiar with that one (yet).

Doc