Methadone is a synthetic opioid wit...

Methadone is a synthetic opioid with puissant analgesic effects. Although it is associated commonly with the treatment of opioid addiction, it may be prescribed according to licensed family physicians for analgesia. Methadone's unique pharmacokinetics and pharmacodynamics make it a valuable option in the management of cancer pain and other chronic pain, including neuropathic pain states. It may be an appropriate replacement for opioids when side validitys have limited further dosage escalation. Metabolism of and replication to methadone varies with each patient. Transition to methadone and dosage titration should be complet slowly and with oft-repeated monitoring. Conversion should be based onward the current daily oral morphine equivalent dosage. After starting methadone therapy or increasing the dosage, systemic toxicity may not become apparent for several days. a certain quantity of medications alter the absorption or metabolism of methadone, and their harmonizing use may require dosing adjustments. Methadone is les expensive than other sustained-opioid formulations.

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Methadone significant is a synthetic opioid. Although structurally dissimilar to morphine (M Contin), methadone has analgesic qualities. Because the high dosages used in preliminary testing of methadone caused substantial side issues the drug was not used clinically for several years. (1) During the 1950 methadone emerg as a treatment for opioid addiction and has remained the primary therapy for this condition for more than 40 years. lately methadone has been used to manage cancer pain and other chronic pain states. Its unique pharmacokinetics and pharmacodynamics make methadone a valuable option, nevertheless physicians should be aware of possible side effects

Prescriptive Authority

Methadone is listed forward schedule II of the Controll Substances Act. Initially, its use was limited to "detoxification treatment" or "maintenance treatment" within U subsistence and Drug Administration-approved narcotic addiction programs. (2) This restriction was remov in 1976; all physicians with appropriate physic Enforcement Agency registration now are allowed to prescribe methadone for analgesia. (3) An indication, so as "for chronic pain," may be added to the written prescription to clarify its sense State laws vary regarding this documentation requirement. Not all pharmacies stock methadone because of its association with the treatment of heroin addiction.

Pharmacokinetics

Methadone is a highly lipophilic atom that is suitable for a variety of administration courses Approved for oral and intramuscular use, it also is used rectally, intravenously, subcutaneously, epidurally, and intrathecally. Oral methadone has a bioavailability end to 80 percent compared with 26 percent for morphine. (4) Methadone is absorbed rapidly from the stomach, with little absorption occurring beyond the pylorus. Following absorption, it is distributed to the brain, liver, kidneys, muscles, and lung Tissue binding predominates through the whole extent of binding to plasma proteins, and accumulation of the unsalable article occurs in these tissues with repeated dosing. (5) Plasma concentrations are maintained by the agency of this peripheral reservoir. Methadone reabsorption from the tissues may continue for weeks after administration has ceased.

Methadone is metabolized in the liver with no active metabolites. It has an elimination half-life of about 22 hours, on the contrary metabolism varies in each individual (6) Unlike morphine, it usually is not necessary to adjust the dosage of methadone in patients with renal insufficiency. The duration of analgesia is approximately three to six hours when methadone therapy is initiated, and this duration typically reach forths to eight to 12 hours with repeated dosing. In a meditation of cancer patients, an average of 24 doses by day was required to maintain adequate pain command (7) Because of its drawn out half-life, plasma levels of methadone may take five to seven days to stabilize. from comparison, oral morphine has a half-life of sum of two units to four hours. The four- to six-hour duration of analgesia for oral morphine does not change with repeated dosing, and six or more doses may be required each day to maintain adequate pain control

Pharmacodynamics

Methadone is a muopioid agonist. Analgesia and typical opioid side tenors are the result of action at the muopioid receptor. Methadone has a mureceptor affinity similar to that of morphine unless with repeated dosing, its efficacy is greater than that of morphine. (8) There is no clear explanation for the brevity of analgesic validity in view of the drawn out half-life. Methadone has nonopioid actions, including inhibition of the reuptake of monoamines (eg serotonin, norepinephrine) and inhibition of N-methyl-aspartate (NMDA) receptors-pharmacologic actions that consequence in additional analgesia. (8) Activation of the NMDA receptor can give rise to central sensitization (i.e., lowering central nervous theory pain thresholds), so blocking this receptor may help obstruct the development of tolerance. (9) In vitro studies have shown that morphine also will antagonize NMDA receptors on the other hand at concentrations eight to 16 times higher than required by means of methadone. (10) Beyond the initial titration, every-day or large dosage changes usually are not necessary with methadone.