Tylenol (acetaminophen) is an analgesic (pain reliever) and antipyretic (fever reducer) used for treating pain and fever associated with many conditions. Tylenol is available in generic form and over-the-counter (OTC). Side effects of Tylenol include: nausea, stomach pain, loss of appetite, itching, rash, headache, dark urine, clay-colored stools, or jaundice (yellowing of skin or eyes). Get medical help right away if you notice symptoms of a rare serious allergic reaction to Tylenol, including: