Acetaminophen, the pain reliever and fever reducer found in Tylenol and many other over-the-counter as well as prescription drugs (such as Lortab), is often hyped for its supposed safety -- specifically because it doesn't cause stomach upset as often as aspirin. However, in recent years, it has become clear that acetaminophen can cause liver damage and, when combined with alcohol, stomach bleeding. Now comes another warning: researchers have linked the drug's use to an increase in asthma and wheezing in both children and adults.