History

Codeine is an alkaloid found in opium and other poppy saps like Papaver bracteatum, the Iranian poppy, in concentrations ranging from 0.3 to 3.0 percent. While codeine can be extracted from opium, most codeine is synthesized from morphine through the process of O-methylation. It was first isolated in 1830 in France by Jean-Pierre Robiquet.

The effects of the Nixon War On Drugs by 1972 or so had caused across-the-board shortages of illicit and licit opiates because of a scarcity of natural opium, poppy straw and other sources of opium alkaloids, and the geopolitical situation was getting less helpful for the United States. After a large percentage of the opium and morphine in the US National Stockpile of Strategic & Critical Materials had to be tapped in order to ease severe shortages of medicinal opiates -- the codeine-based antitussives in particular -- in late 1973, researchers were tasked with and quickly succeeded in finding a way to synthesize codeine and its derivatives and precursors from scratch from petroleum or coal tar using a process developed at the United States' National Institutes of Health.

Numerous codeine salts have been prepared since the drug was discovered. The most commonly used are the hydrochloride (freebase conversion ratio 0.805), phosphate (0.736), sulphate (0.859) and citrate (0.842). Others include a salicylate NSAID, codeine salicylate (0.686), and at least four codeine-based barbiturates, the cyclohexenylethylbarbiturate (0.559), cyclopentenylallylbarbiturate (0.561), diallylbarbiturate (0.561), and diethylbarbiturate (0.619).

Pharmacology

Codeine is considered a prodrug, since it is metabolised in vivo to the primary active compounds morphine and codeine-6-glucuronide. Roughly 5-10% of codeine will be converted to morphine, with the remainder either free, conjugated to form codeine-6-glucuronide (~70%), or converted to norcodeine (~10%) and hydromorphone (~1%). It is less potent than morphine and has a correspondingly lower dependence-liability than morphine. Like all opioids, continued use of codeine induces physical dependence and can be psychologically addictive. However, the withdrawal symptoms are relatively mild and as a consequence codeine is considerably less addictive than the other opiates.

A dose of approximately 200 mg (oral) of codeine must be administered to give analgesia approximately equivalent to 30 mg (oral) of morphine (Rossi, 2004). However, codeine is generally not used in single doses of greater than 60 mg (and no more than 240 mg in 24 hours). When analgesia beyond this is required, stronger opioids such as hydrocodone or oxycodone are favored. Because codeine needs to be metabolized to an active form, there is a ceiling effect around 400-450 mg. This low ceiling further contributes to codeine being less addictive than the other opiates. The ceiling dose can be more accurately calculated by using 7 mg per 1 kg of bodyweight for males and 6 mg per 1 kg of bodyweight for females, taking into consideration an 18.5-25 BMI bodyweight to not over or under calculate in cases of obese or underweight people (this rule does not take into consideration the usage of other CYP2D6 inhibiting drugs, alcohol or naturally low or high enzyme presence though a 25% enzyme occupancy reduction can prove more accurate results). Example: A 85kg male, bearing a height of 180cm is 5kg overweight, therefore the ceiling dose can be calculated by 7mg x (85-5)kg = 560mg and for a female of the same height and weight, 6mg x (85-5)kg = 480mg. 25% reduction to consider momentary enzyme occupancy, (560x0.75) putting the ceiling limit at 420mg. Total CYP2D6 saturation or exceeding the ceiling dose often results in excessive histamine release to combat the yet to be processed codeine which results in swelling of bodily areas such as the genitals, cheeks, hands and feet often mimicking an adverse or allergic reaction despite subsidence with time.