Codeine (INN) is an opioid used for its analgesic, antitussive and antidiarrheal properties. It is commonly marketed as the phosphate salt 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.

Indications
Approved indications for codeine include:
cough - though its efficacy has been disputed (Schroeder & Fahey, 2001)
diarrhea
mild-to-moderate pain
Codeine is sometimes marketed in combination preparations with paracetamol (co-codamol), aspirin (co-codaprin) or ibuprofen; for mild-to-moderate pain. These combinations provide greater pain relief than either agent used singly.

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 over-the-counter (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; It is a Class B Drug.

Pharmacokinetics
Codeine is considered a prodrug, since it is metabolized 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 200mg (oral) of codeine must be administered to give equivalent analgesia to 30mg (oral) of morphine (Rossi, 2004). It is not used, however, in single doses of greater than 60mg (and no more than 240mg 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.

Pharmacology
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.

Adverse effects
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)

With prolonged use, tolerance develops to many of the effects of opioids such as codeine, including the adverse effects.

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.
The source of this article is Wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL

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