I just came across a study in the Journal of the American Medical Assoication from a few years ago showing that taking Acetaminophen daily causes liver damage in just two weeks! Most people understand that taking pain relievers for extended periods of time (years for example) would cause liver, and other organ damage. But I don't think anyone realized how soon liver damage can occur.
So when ever you are having any type of back pain for more then just a couple of days, you need to have it checked by a chiropractor, and get the source of the pain fixed. Remember, the pain medication only masks the pain, and does nothing to solve the source of the problem. Do not take Acetaminophen for any extended periods of time to resolve your back pain - think of your liver!
http://jama.ama-assn.org/cgi/content/abstract/296/1/87