We are not sure. Acetaminophen or Tylenol is great for headache, pain and fever reduction but does very little for inflammation. Tylenol falls under a broad category called no opioid analgesics. Other drugs included in this subclass include aspirin and non-steroidal anti-inflammatory drugs like ibuprofen, Naprosyn and Celebrex. More is known about how these other drugs work. They inhibit prostaglandin formation by inhibiting an enzyme called cyclooxygenase and they work throughout the body. Prostaglandins serve protective functions but they can also produce pain, inflammation and fever. Tylenol seems to block prostaglandin production in the brain and spinal cord. This is why Tylenol works great on a headache but not so well in bringing down swelling in a sprained ankle.