Analgesics

Definition

Analgesics are medicines that relieve pain .

Description

Analgesics are those drugs whose primary purpose is pain relief. The primary classes of analgesics are the narcotics, including additional agents that are chemically based on the morphine molecule but have minimal abuse potential; nonsteroidal anti-inflammatory drugs (NSAIDs) including the salicylates; and acetaminophen . Other drugs, notably the tricyclic antidepressants and anti-epileptic agents, such as gabapentin, have been used to relieve pain, particularly neurologic pain, but are not routinely classified as analgesics. Analgesics provide symptomatic relief but have no effect on causation, although clearly the NSAIDs, by virtue of their dual activities as pain relievers and anti-inflammatories, may be beneficial in both regards.

Description

Pain has been classified as "productive" and "non-productive." While this distinction has no physiologic meaning, it may serve as a guide to treatment. Productive pain has been described as a warning of injury and so may be both an indication of need for treatment and a guide to diagnosis. Non-productive pain by definition serves no purpose either as a warning or diagnostic tool.

Although pain syndromes may be dissimilar, the common factor is a sensory pathway from the affected organ to the brain. Analgesics work at the level of the nerves, either by blocking the signal from the peripheral nervous system or by distorting the interpretation by the central nervous system. Selection of an appropriate analgesic is based on consideration of the risk-benefit factors of each class of drugs, based on type of pain, severity of pain, and risk of adverse effects. Traditionally, pain has been divided into two classes, acute and chronic, although severity and projected patient survival are other factors that must be considered in drug selection.

Acute pain

Acute pain is self limiting in duration and includes post-operative pain, pain of injury, and childbirth . Because pain of these types is expected to be short term, the long-term side effects of analgesic therapy may routinely be ignored. Thus, these patients may safely be treated with narcotic analgesics without concern for their addictive potential, or NSAIDs with only limited concern for their ulcerogenic (ulcer-causing) risks. Drugs and doses should be adjusted based on observation of healing rate, switching patients from high to low doses and from narcotic analgesics to non-narcotics when circumstances permit.

An important consideration of pain management in severe pain is that patients should not be subject to the return of pain. Analgesics should be dosed adequately to assure that the pain is at least tolerable and frequently enough to avoid the anxiety that accompanies the anticipated return of pain. Generally analgesics should not be dosed on an as-needed basis but should be administered often enough to assure constant blood levels of analgesic. This applies to both the narcotic and non-narcotic analgesics.

Chronic pain

Chronic pain, pain lasting over three months and severe enough to impair function, is more difficult to treat, since the anticipated side effects of the analgesics are more difficult to manage. In the case of narcotic analgesics this means the addiction potential, as well as respiratory depression and constipation . For the NSAIDs, the risk of gastric ulcers may be dose limiting. While some classes of drugs, such as the narcotic agonist/antagonist drugs bupronophine, nalbuphine, and pentazocine, and the selective COX-2 inhibitors celecoxib and rofecoxib represent advances in reduction of adverse effects, they are still not fully suitable for long-term management of severe pain. Generally, chronic pain management requires a combination of drug therapy, life-style modification, and other treatment means.

Narcotic analgesics

The narcotic analgesics, also termed opioids, are all derived from opium. The class includes morphine, codeine, and a number of semi-synthetics including meperidine (Demerol), propoxyphen (Darvon), and others. The narcotic analgesics vary in potency, but all are effective in treatment of visceral pain when used in adequate doses. Adverse effects are dose related. Because these drugs are all addictive, they are controlled under federal and state laws. A variety of dosage forms are available, including oral solids, liquids, intravenous and intrathecal injections, and transcutaneous patches.

NSAIDs are effective analgesics even at doses too low to have any anti-inflammatory effects. There are a number of chemical classes, but all have similar therapeutic effects and side effects. Most are appropriate only for oral administration; however, ketorolac (Toradol) is appropriate for injection and may be used in moderate to severe pain for short periods.

Three new NSAIDs, celecoxib, rofecoxib, and valdecoxib may reduce the risk of gastric ulcers in long-term use for adults and have been widely advertised. As of 2004 these drugs had not been properly tested in children, and even in adults, their advantages were not well established. These drugs should not be given to infants and are not well documented for use in older children.

Acetaminophen is a non-narcotic analgesic with no anti-inflammatory properties. It is appropriate for mild to moderate pain. Although the drug is well tolerated in normal doses, it may have significant toxicity at high doses. Because acetaminophen is largely free of side effects at therapeutic doses, it has been considered the first choice for mild pain, including that of osteoarthritis.

General use

Appropriate dosage varies by drug and should consider the type of pain, as well as other risks associated with patient age and condition. For example, narcotic analgesics should usually be avoided in patients with a history of substance abuse but may be fully appropriate in patients with cancer pain. Similarly, because narcotics are more rapidly metabolized in patients who have used these drugs for a long period, higher than normal doses may be needed to provide adequate pain management.

Precautions

Narcotic analgesics may be contraindicated in patients with poor respiratory function. NSAIDS should be used with care in patients with insufficient kidney function or coagulation disorders . NSAIDs are contraindicated in patients who are allergic to aspirin.

Side effects

Parents of children taking analgesics should review adverse effects of each drug individually. Drugs within a class may vary in their frequency and severity of adverse effects.

The primary adverse effects of the narcotic analgesics are addiction, constipation, and poor respiratory function. Because narcotic analgesics stimulate the production of enzymes that cause the metabolism of these drugs, patients on narcotics for a prolonged period may require increasing doses. This physical tolerance is not the same thing as addiction and is not a reason for withholding medication from patients in severe pain.

NSAIDs may cause kidney problems. Gastrointestinal discomfort is common, although in some cases, these drugs may cause ulcers without the prior warning of gastrointestinal distress. NSAIDs may cause blood to clot less readily, although not to the same extent as if seen with aspirin.

Interactions

Parents should study information on interactions for specific drugs their children are taking.

Analgesics will interact with other drugs that have similar side effects. Nonsteroidal anti-inflammatory drugs should be used with care with other drugs that may cause stomach upset, such as aspirin. Narcotic analgesics should be used with care when taken in combination with drugs that inhibit respirations, such as the benzodiazepines.

Parental concerns

Regarding acetaminophen, parents should never confuse baby formulations, which are high concentration, with children's formulas. The infant formulas are meant to be given by the drop, never by the teaspoonful. Children's liquids are for teaspoonful dosing. Parents must read labels carefully and use the appropriate measure.

Aspirin should never be given to children under the age of 16 who have chickenpox or influenza , because children who have received aspirin for these conditions seem to have a higher than expected frequency of developing Reye's syndrome . High dose aspirin may be given to children for treatment of rheumatism, but this should only be done under medical supervision.

Regarding narcotics, although addiction is a concern when narcotic analgesics are used, this concern is not a problem when the medications are given appropriately. When a child is in severe pain, these pain relievers should not be withheld.

See also Acetaminophen ; Nonsteroidal anti-inflammatory drugs ; Pain management .

Resources

BOOKS

Beers, Mark H., and Robert Berkow, eds. The Merck Manual , 2nd home ed. West Point, PA: Merck & Co., 2004.

Mcevoy, Gerald, et al. AHFS Drug Information 2004. Bethesda, MD: American Society of Healthsystems Pharmacists, 2004.

Siberry, George K., and Robert Iannone, eds. The Harriet Lane Handbook , 15th ed. Philadelphia: Mosby, 2000.

PERIODICALS

Losek, J. D. "Acetaminophen dose accuracy and pediatric emergency care." Pediatric Emergency Care 20 (May 2004): 285–8.

Rupp, T., and K. A. Delaney. "Inadequate analgesia in emergency medicine." Annual Emergency Medicine 43 (April 2004): 494–503.

ORGANIZATIONS

American Academy of Pediatrics. 141 Northwest Point Boulevard Elk Grove Village, IL 60007-1098. Web site: http://www.aap.org.

American Pain Society. 47000 W. Lake Ave., Glenview, IL 60025. Web site: http://www.ampainsoc.org/.

WEB SITES

"Pediatric Chronic Pain." American Pain Society , January-February 2001. Available online at http://www.ampainsoc.org/pub/bulletin/jan01/posi1.htm (accessed December 19, 2004).

OTHER

Pediatric Pain [Internet links]. Available online at http://www.painandhealth.org/pediatric-links.htm (accessed December 19, 2004).

Samuel Uretsky, PharmD