•Acetaminophen: Acetaminophen appears to act within the CNS. Acetaminophen appears to increase the pain threshold by inhibiting central cyclooxygenase, an enzyme involved in prostaglandin (PG) synthesis (see Acetaminophen monograph). Acetaminophen inhibits both isoforms of central cyclooxygenase, COX-1 and COX-2. Acetaminophen does not inhibit PG synthesis in peripheral tissues, which is the reason for its lack of peripheral anti-inflammatory effects.