This might cause you to think twice before popping pills over a little headache. Acetaminophen, one of the most common medicines used in fever and pain drugs like Tylenol, could trigger dangerous skin reactions in some people, the Food and Drug Administration said Thursday. The reactions typically start with flu-like symptoms, but could lead to extensive damage to skin tissue and could be fatal. The most serious reactions are Stevens-Johnson Syndrome and toxic epidermal necrolysis. The FDA is requiring manufacturers to place warning labels on prescription medications that contain acetaminophen, and potentially on over-the-counter drugs as well.