Tylenol, which contains acetaminophen, affects the liver. The liver breaks down almost all of the acetaminophen in Tylenol, and during this process, a toxic chemical called NAPQI forms. If the liver cant properly process all of the NAPQI, it can build up and poison liver cells, leading to liver damage or failure. However, occasional use of Tylenol at recommended doses is generally safe for most people. The kidneys dont play a big role in breaking down Tylenol, but they clear it from the body after the liver detoxifies the medication, so Tylenol doesnt have much effect on the kidneys.