Tool Module: Anesthesics and Analgesics
Pain is one of the primary manifestations of injuries and of acute and chronic illnesses. Even though natural healing processes can alleviate pain and eventually make it disappear, people have been trying since time immemorial to relieve it by means of substances extracted from plants (such as morphine) or, more recently, produced in the laboratory.
Growing scientific understanding of the receptors and neurotransmitters involved in the body’s nociceptive (pain-sensing) pathways has led to the development of new medications that prevent or diminish pain. These molecules act by binding to specific receptors that they activate or block at various locations along these pathways. These various kinds of medications can thus modify the transmission of pain both in the peripheral nervous system and in the spinal cord and the brain.
There are two main classes of medications that modulate the perception of pain: anesthetics and analgesics.
Anesthetics are used to reduce activity of the nervous system so as to prevent pain during surgical operations.
General anesthetics are compounds that act on the brain by lowering its level of alertness until the individual loses consciousness. Because pain is a conscious process, it disappears when the anesthetized brain thus falls asleep. But this is an artificial form of sleep, very different from natural sleep. The source of the pain is still there, and so are the physiological reflexes that accompany it, but the individual is no longer aware of them.
Local or regional anesthetics are used to put only the nerves in a certain part of the body “to sleep”. The most familiar examples may be the local anesthetic that your dentist uses to put the nerves in your teeth to sleep so that no pain reaches your brain while he or she is drilling, and the epidural anesthesia used to relieve the pain of childbirth.
Analgesics are a class of medications used to alleviate or eliminate pain. They can be used to relieve post-operative pain, but their most common application is to virtually any kind of chronic or acute pain. Analgesics are also referred to as antipain medications, or painkillers.
Unlike general anesthetics, analgesics do not cause loss of consciousness. Each type of analgesic medication requires a specific amount of time to take effect and has a specific duration of action. Some analgesics take effect in just a few seconds, but their effects last for only a few minutes. Others, such as morphine, take effect more slowly (in about 15 minutes), but their effects are longer-lasting (3 to 4 hours).
Two of the most commonly used categories of analgesics are opiates and nonsteroidal anti-inflammatory drugs (NSAIDs).
Opiates included natural opiates—substances extracted from opium poppies, such as morphine—and their synthetic derivatives, such as meperidine (Demerol) and methadone. Opiates have long been and remain the type of medication most often used to relieve moderate to severe pain. The higher the dosage, the more effective they are, but the more their undesirable side effects become disabling and dangerous as well. The most common of these side effects are constipation, sleepiness, and depressed breathing.
Opiates act on one or more of the main opioid receptors and their subtypes in the brain. Some of the undesirable side-effects of opiates are associated with the stimulation of certain specific receptor subtypes. Researchers are therefore trying to synthesize opiates that do not target these subtypes, but only those receptors that produce the desired analgesic effect.
Nonsteroidal anti-inflammatory drugs (NSAIDs) are so designated to distinguish them from steroids (glucocorticoids), which also have significant anti-inflammatory properties. NSAIDs have an analgesic effect that relieves light to moderate pain. The two best known NSAIDs are aspirin and ibuprofen.
Aspirin (acetylsalicylic acid) is the most popular medication in the world. It was first marketed in 1899, by Bayer, a German company. Aspirin acts by inhibiting cyclooxygenase, an enzyme that is important for the synthesis of prostaglandins, which are derived from arachidonic acid, a molecule released when body tissue is injured. Prostaglandins depolarize nociceptive neurons, thus making them more sensitive and causing people to be more sensitive to pain. By blocking the production of the enzyme that manufactures these prostaglandins, aspirin thus reduces the painful inflammation that they cause.
NSAIDs such as aspirin have a number of other effects, some beneficial, others harmful. NSAIDs lower fever, slow blood coagulation, can cause stomach ulcers, and have toxic effects on the kidneys and liver.
Paracetamol, also called acetaminophen, is another analgesic (painkilling) and antipyretic (fever-lowering) medication. It has been on the market since 1950, and its strength is comparable to that of aspirin. Unlike aspirin, acetaminophen has practically no anti-inflammatory effect and no effect on the clumping of platelets that results in blood clotting. Hence it cannot be prescribed, as aspirin can, to reduce the risks of heart attacks and strokes caused by clotting in blood vessels. Acetaminophen is the active ingredient in numerous medications sold under various names (Tylenol or Panadol in Canada and the United States, and Doliprane, Dafalgan, or Efferalgan in Europe). It has no serious side effects when taken in the recommended doses, but overdoses can be highly toxic to the liver.
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