analgesic

analgesic

[an-l-jee-zik, -sik]
analgesic, any of a diverse group of drugs used to relieve pain. Analgesic drugs include the nonsteroidal anti-inflammatory drugs (NSAIDs) such as the salicylates, narcotic drugs such as morphine, and synthetic drugs with morphinelike action such as meperidine (Demerol) and propoxyphene (Darvon). Aspirin and other NSAIDs (e.g., acetaminophen, ibuprofen, and naproxen) reduce fever and inflammation as well as relieve pain. Narcotic analgesics and the morphinelike synthetic drugs depress the central nervous system and alter the perception of pain. They are used to alleviate pain not relieved by the NSAIDs. NSAIDs and other analgesics are also used in combination, as in Tylenol with codeine and Darvocet (Darvon and acetaminophen). Recently, patient-controlled analgesic techniques have been introduced, in which patients have the option of injecting small quantities of narcotic type analgesics to control their own pain. Microprocessor-controlled injections may be made through intravenous catheters, or through a catheter into the epidural (covering of the spinal cord) area. In addition to analgesic drugs, various techniques, such as acupuncture, hypnosis (see hypnotism), and biofeedback, are used to alleviate pain.

Drug that produces analgesia (see analgesic), narcosis (stupor or sleep), and drug addiction. In most people narcotics also produce euphoria. Those that occur naturally in the opium poppy, notably morphine, have been used since ancient Greek times. The main therapeutic use of narcotics is for pain relief. Most countries limit the production, sale, and use of narcotics because of their addictive properties and detrimental effects and the incidence of drug abuse. With the development in the 19th century of the hypodermic needle and of heroin, five to 10 times as potent as morphine, the use and abuse of narcotics increased dramatically. A narcotic overdose can cause central nervous system depression, respiratory failure, and death.

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Drug that relieves pain without blocking nerve impulse conduction or markedly altering sensory function (see nervous system). Two classes are defined by the type of pain-relieving action. Opioids (opiates and synthetic narcotics; see opium) act on brain receptors to inhibit pain impulses. They may be used for short- or long-term pain relief, usually by prescription, but carry a risk of drug addiction. Nonopioids, used mostly for short-term relief and modest pain, are available without prescription. They include NSAIDs (including aspirin and ibuprofen) and acetaminophen; all act by inhibiting synthesis of prostaglandins, molecules involved in the peripheral perception of pain.

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An analgesic (colloquially known as a painkiller) is any member of the diverse group of drugs used to relieve pain (achieve analgesia). The word analgesic derives from Greek an- ("without") and -algia ("pain"). Analgesic drugs act in various ways on the peripheral and central nervous systems; they include paracetamol (acetaminophen), the non-steroidal anti-inflammatory drugs (NSAIDs) such as the salicylates, narcotic drugs such as morphine, synthetic drugs with narcotic properties such as tramadol, and various others.

In choosing analgesia, the severity and response to other medication determines the choice of agent; the WHO pain ladder, originally developed in cancer-related pain, is widely applied to find suitable drugs in a stepwise manner. The choice of analgesia is also determined by the type of pain: for neuropathic pain, traditional analgesia is less effective, and there is often benefit from classes of drugs that are not normally considered analgesics, such as tricyclic antidepressants and anticonvulsants.

The major classes

Paracetamol and NSAIDs

The exact mechanism of action of paracetamol/acetaminophen is uncertain, but it appears to be acting centrally. Aspirin and the other non-steroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenases, leading to a decrease in prostaglandin production. This reduces pain and also inflammation (in contrast to paracetamol and the opioids).

Paracetamol has few side effects and is regarded as very safe, although excessive doses can lead to kidney and liver damage in the form of analgesic nephropathy and paracetamol hepatotoxicity, respectively. NSAIDs predispose to peptic ulcers, renal failure, allergic reactions, and occasionally hearing loss, and they can increase the risk of hemorrhage by affecting platelet function. The use of aspirin in children under 16 suffering from viral illness may contribute to Reye syndrome.

COX-2 inhibitors

These drugs have been derived from NSAIDs. The cyclooxygenase enzyme inhibited by NSAIDs was discovered to have at least 2 different versions: COX1 and COX2. Research suggested that most of the adverse effects of NSAIDs were mediated by blocking the COX1 (constitutive) enzyme, with the analgesic effects being mediated by the COX2 (inducible) enzyme. The COX2 inhibitors were thus developed to inhibit only the COX2 enzyme (traditional NSAIDs block both versions in general). These drugs (such as rofecoxib and celecoxib) are equally effective analgesics when compared with NSAIDs, but cause less gastrointestinal hemorrhage in particular. However, post-launch data indicated increased risk of cardiac and cerebrovascular events with these drugs due to an increased likelihood of clotting in the blood due to a decrease in the production of protoglandin around the platelets causing less clotting factor to be released, and rofecoxib was subsequently withdrawn from the market. The role for this class of drug is debated.

Opiates and morphinomimetics

Morphine, the archetypal opioid, and various other substances (e.g. codeine, oxycodone, hydrocodone, diamorphine, pethidine) all exert a similar influence on the cerebral opioid receptor system. Tramadol and buprenorphine are thought to be partial agonists of the opioid receptors. Dosing of all opioids may be limited by opioid toxicity (confusion, respiratory depression, myoclonic jerks and pinpoint pupils), but there is no dose ceiling in patients who tolerate this.

Opioids, while very effective analgesics, may have some unpleasant side-effects. Up to 1 in 3 patients starting morphine may experience nausea and vomiting (generally relieved by a short course of antiemetics). Pruritus (itching) may require switching to a different opioid. Constipation occurs in almost all patients on opioids, and laxatives (lactulose, macrogol-containing or co-danthramer) are typically co-prescribed.

When used appropriately, opioids and similar narcotic analgesics are otherwise safe and effective, however risks such as addiction and the body becoming used to the drug (tolerance) can occur. The effect of tolerance means that drug dosing may have to be increased if it is for a chronic disease this is where the no ceiling limit of the drug comes into play. However what must be remembered is although there is no upper limit there is a still a toxic dose even if the body has become used to lower doses.

Specific agents

In patients with chronic or neuropathic pain, various other substances may have analgesic properties. Tricyclic antidepressants, especially amitriptyline, have been shown to improve pain in what appears to be a central manner. The exact mechanism of carbamazepine, gabapentin and pregabalin is similarly unclear, but these anticonvulsants are used to treat neuropathic pain with modest success.

Specific forms and uses

Combinations

Analgesics are frequently used in combination, such as the paracetamol and codeine preparations found in many non-prescription pain relievers. They can also be found in combination with vasoconstrictor drugs such as pseudoephedrine for sinus-related preparations, or with antihistamine drugs for allergy sufferers.

The use of paracetamol, as well as aspirin, ibuprofen, naproxen, and other NSAIDS concurrently with weak to mid-range opiates (up to about the hydrocodone level) has been shown to have beneficial synergistic effects by combatting pain at multiple sites of action—NSAIDs reduce inflammation which, in some cases, is the cause of the pain itself while opiates dull the perception of pain—thus, in cases of mild to moderate pain caused in part by inflammation, it is generally recommended that the two be prescribed together.

Topical or systemic

Topical analgesia is generally recommended to avoid systemic side-effects. Painful joints, for example, may be treated with an ibuprofen- or diclofenac-containing gel; capsaicin also is used topically. Lidocaine, an anesthetic, and steroids may be injected into painful joints for longer-term pain relief. Lidocaine is also used for painful mouth sores and to numb areas for dental work and minor medical procedures.

Psychotropic agents

Tetrahydrocannabinol (THC) and some other cannabinoids, either from the Cannabis sativa plant or synthetic, have analgesic properties, although the use of cannabis derivatives is illegal in many countries. Other psychotropic analgesic agents include ketamine (an NMDA receptor antagonist), clonidine and other α2-adrenoreceptor agonists, and mexiletine and other local anaesthetic analogues.

Atypical and/or adjuvant analgesics

Orphenadrine, cyclobenzaprine, scopolamine, atropine, gabapentin, first-generation antidepressants and other drugs possessing anticholinergic and/or antispasmodic properties are used in many cases along with analgesics to potentiate centrally acting analgesics such as opioids when used against pain especially of neuropathic origin and to modulate the effects of many other types of analgesics by action in the parasympathetic nervous system. Dextromethorphan has been noted to slow the development of tolerance to opioids and exert additional analgesia by acting upon the NMDA receptors; some analgesics such as methadone and ketobemidone and perhaps piritramide have intrinsic NMDA action. High-alcohol liquor has been used in the past as an agent for dulling pain, due to the CNS depressant effects of ethyl alcohol, a notable example being the American Civil War. However, the ability of alcohol to "kill pain" is said to be inferior to virtually all analgesics used today (e.g. morphine, codeine). As such, the idea of alcohol for analgesia is generally regarded as being as primitive as the practice in virtually all industrialized countries today.

The use of adjuvant analgesics is an important and growing part of the pain-control field and new discoveries are made practically every year. Many of these drugs combat the side effects of opioid analgesics, an added bonus. For example, antihistamines including orphenadrine combat the release of histamine caused by many opioids, methylphenidate, caffeine, ephedrine, dextroamphetamine, and cocaine work against heavy sedation and may elevate mood in distressed patients as do the antidepressants. A well-accepted benefit of THC to chronic pain patients on opioids is its superior anti-nauseant action. Some think it would make more sense to use the synthetic THC capsule (trade name Marinol), which is administered orally. However, in patients suffering from nausea, the swallowing of the capsule itself may provoke vomiting. Likewise, the use of medicinal cannabis remains a debated issue.

Addiction

In the United States in recent years, there has been a wave of new addictions to prescription narcotics such as oxycodone (such as OxyContin, or with acetaminophen, as Percocet) and hydrocodone (commonly prescribed with acetaminophen, as in Vicodin, Lortab etc.) when available in pure formulations as opposed to combined with other medications (as in Percocet which contains both oxycodone and acetaminophen/paracetamol).

See also

References

External links

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