Codeine (INN) or methylmorphine is an opioid used for its analgesic, antitussive and antidiarrheal properties. It is marketed as the salts codeine sulfate and codeine phosphate.
Codeine is an alkaloid found in opium in concentrations ranging from 0.7 to 2.5 percent. While codeine can be extracted from opium, most codeine used in the United States is synthesized from morphine through the process of O-methylation.
Approved indications for codeine include:
Cough, though its efficacy has been disputed (Schroeder & Fahey, 2001)
Mild to moderate pain
Codeine is sometimes marketed in combination preparations with paracetamol (acetaminophen) as co-codamol, with aspirin co-codaprin or with ibuprofen. These combinations provide greater pain relief than either agent used singly (q.v. Drug Synergy).
In the United States, codeine is regulated by the Controlled Substances Act. It is a Schedule II controlled substance for pain-relief products containing codeine alone. In combination with aspirin or acetaminophen (paracetamol) it is listed as Schedule III. Codeine is also available outside the United States as an over-the-counter medication (Schedule V) in liquid cough-relief formulations. Internationally, codeine is a Schedule II drug under the Single Convention on Narcotic Drugs.
In the United Kingdom, codeine is regulated by the Misuse of Drugs Act 1971; it is a Class B Drug.
In Australia and Canada, codeine is regulated, however it is available without prescription in combination preparations from licensed pharmacists in doses up to 15 mg/tablets.
Codeine is considered a prodrug, since it is metabolised in vivo to the principal active analgesic agent morphine. It is, however, less potent than morphine since only about 10% of the codeine is converted. It also has a correspondingly lower dependence-liability than morphine.
Theoretically, a dose of approximately 200 mg (oral) of codeine must be administered to give equivalent analgesia to 30 mg (oral) of morphine (Rossi, 2004). It is not used, however, in single doses of greater than 60mg (and no more than 240 mg in 24 hours) since there is a ceiling effect.
The conversion of codeine to morphine occurs in the liver and is catalysed by the cytochrome P450 enzyme CYP2D6. Approximately 6–10% of the Caucasian population have poorly functional CYP2D6 and codeine is virtually ineffective for analgesia in these patients (Rossi, 2004). Many of the adverse effects, however, are still experienced. Also, some medications are CYP2D6 inhibitors and reduce or even completely eliminate the efficacy of codeine. The most notorious of these are the selective serotonin reuptake inhibitors, such as fluoxetine (Prozac) and citalopram (Celexa).
Main article: opioid receptor
Codeine itself has weak affinity for the μ-opioid receptor. Its principal analgesic actions are mediated by the affinity of morphine for the μ-opioid receptor, though other therapeutic and adverse effects are produced by activation of other opioid receptors.
Common adverse drug reactions (ADRs) associated with the use of codeine include: Itching, nausea, vomiting, drowsiness, dry mouth, miosis, orthostatic hypotension, urinary retention and constipation. (Rossi, 2004)
Tolerance to many of the effects of codeine develop with prolonged use, including therapeutic effects. The rate at which this occurs develops at different rates for different effects, with tolerance to the constipation-inducing effects developing particularly slowly for instance.
A potentially serious ADR, as with other opioids, is respiratory depression. This depression is dose-related and is the mechanism for the potentially fatal consequences of overdose.
Codeine is often used as a recreational drug. This may be due to its easy availability over-the-counter or on prescription in combination products (which, in certain countries, are scheduled lower than codeine as a single-agent). People use it in order to obtain the euphoric effects associated with use of opioids.
In certain areas of the United States; more specifically Texas, codeine syrup form is called Lean. It is commonly mixed with alcohol, or a blunt and smoked. It has been said this is done because it makes you "lean".
In some countries, cough syrups and tablets containing codeine are available without prescription; people will frequently purchase it from multiple pharmacies so as not to incur suspicions. It is reported that in France, 95% of the consumption of Néo-codion cough preparation, containing codeine, cannot be attributed to medical use, but is rather used as a substitute for heroin.
In the United Kingdom, people purchase tablets which combine codeine and paracetamol (acetaminophen), and consume these at higher-than-recommended doses, without apparent concern of the hepatotoxicity associated with large doses of paracetamol. Some may try to extract the codeine from the paracetamol through various methods, the most common and simplest being the cold water extraction.
While the combination of codeine with paracetamol, at higher-than-recommended doses, can possibly cause hepatotoxicity (liver damage), combination with ibuprofen can result in kidney problems/failure and additional stomach pain and nausea and combination with aspirin can lead to internal hemorrhaging, particularly gastrointestinal hemorrhage.
Certain codeine products are encountered on the illicit market, frequently in combination with carisoprodol. Combinations of codeine and glutethimide (Doriden) used to be fairly commonplace, but are almost unheard of today, due to the withdrawal of glutethimide products from the marketplace in the US and almost all other countries.
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