ANALGESIC

drug that relieves pain without blocking the conduction of nerve impulses or markedly altering the function of the sensory apparatus. Analgesics are classified by the mechanism of their pain-relieving action as opioids (opiates and similar synthetic compounds), which act on receptors in the brain to inhibit pain impulses, or nonopioids (nonsteroidal anti-inflammatory drugs, or NSAIDs, as well as acetaminophen and phenacetin), which inhibit the synthesis of prostaglandins. Opioid analgesics may be used for either short- or long-term pain relief, though drug tolerance and physical dependence develop with prolonged use. Nonopioid analgesics are generally used for the short-term relief of mild to moderate pain and are often self-administered. Most of the commonly used mild nonopioid analgesics are derived from three compounds, all first discovered in the 19th century: salicylic acid, pyrazolone derivatives, and phenacetin (or acetophenetidin). The most commonly used analgesic is aspirin (q.v.), or acetylsalicylic acid, which reduces fever and relieves inflammation, as well as lessening pain. Taken in overdose, aspirin can cause deafness, ringing in the ears, diarrhea, nausea, and headache, which disappear when the dose is reduced or stopped. Continual use of aspirin often causes irritation to the stomach wall with associated pain, nausea, vomiting, and bleeding. Aspirin and other salicylates have been linked to Reye's syndrome (q.v.) when used to reduce fever during viral illnesses, and thus their use to treat children is discouraged. Of the newer NSAIDs, the most commonly used is ibuprofen, introduced in the 1970s. It shares many aspirin-like properties and has similar uses, though it causes fewer side effects and is better tolerated by most individuals. Acetaminophen is another popular mild analgesic and antipyretic. Chemically it is a phenacetin derivative and is a suitable alternative to aspirin for patients who develop severe symptoms of stomach irritation, because it is not as harmful to the gastrointestinal tract. It does not possess equal anti-inflammatory activity, however, and so is ineffective in treating rheumatoid arthritis. Acetaminophen and similar drugs can cause kidney and liver damage if taken in overdose. The pyrazolone analgesics (such as phenylbutazone) have effects similar to those of aspirin; they were commonly used to treat rheumatoid arthritis, but for most other applications they have been replaced by NSAIDs.