Acetaminophen is a very commonly used pain reliever and fever reducer. It is available over the counter, or by prescription, either by itself or in combination with other medications. Most of us are aware of Tylenol, a commonly used brand of acetaminophen in the United States. It works by reducing the synthesis of prostaglandins, which are hormones that play a role in pain and inflammation. In otherwise healthy people, acetaminophen is generally safe when taken as directed. The most common side effect of acetaminophen is mild stomach pain. Other possible side effects include nausea, vomiting, diarrhea, constipation, and skin rash. These side effects are usually mild and go away on their own.