Using CodeineCodeine, like other opiates, is used recreationally for its euphoric properties. As codeine is available OTC in a few large countries, users there usually buy packs of codeine-containing products and extract the codeine. Because pills with codeine often also contain acetaminophen which, even in moderate overdose, can cause renal failure, it is highly advised to remove the acetaminophen. The recommended daily maximum intake of acetaminophen is 3000-4000mg (the FDA now recommends 3000mg per day) with no more than 1000mg at a time. Individual doses should be separated by at least four hours, better six. Large doses of acetaminophen, especially chronic, can cause liver failure. Several cases have been recorded of an overdose at merely 4000mg on one day.
Codeine can be quite easily extracted through a process called cold-water extraction: codeine is highly soluble in warm and cold water, while acetaminophen is MUCH less soluble in warm and cold water, and even less soluble as temperature decreases. Pills are usually crushed into a fine powder, mixed with enough warm water, and put in the fridge and/or freezer to cool it down. Most of the acetaminophen will precipitate out (form a solid, while the codeine stays dissolved in the water), and the solution/precipitate is then filtered. The resulting liquid contains nearly all of the codeine and much less acetaminophen. Measures can also be taken to remove caffeine, as this is usually also present in codeine-containg products to prevent over-sedation and drowsiness. One must keep in mind, however, that because acetaminophen is not completely insoluble in water, some still remains (approximately 14mg/ml of water).
Codeine is also found in cough medicines, however in this case, in addition to being combined with acetaminophen, it usually also contains guaifinesin, an expectorant, and an abundance of sugars and binders. Too much guaifinesin causes vomiting and other more serious side effects. Codeine should not be abused in this form for any reason, and it is not time nor cost effective to attempt to extract the codeine.
A great thread on CWE can be found here. There is also a wiki article specifically about CWE here
Ways of administrationCodeine is almost exclusively administered orally, for general pain relief, antitussion, and to achieve opiate-induced euphoria. Because of the almost exclusive availability of codeine in products containing acetaminophen and other active and inactive products and binders, it is very unwise to attempt any other route of administration. In addition, unless lab-grade codeine is obtained, injection should never be attempted. Codeine is available for injection, but it is unlikely one will ever encounter it because of the more available and effective products available for injection such as morphine, or other pain relievers which don't need to be injected.
Effects of CodeineCodeine, like other opiates, has analgesic, antitussive, and antidiarrheal properties. The most common effects of codeine are euphoria, constipation, respiratory depression, sedation, light-headedness, drowsiness, nausea, urinary retention, miosis, and erectile dysfunction (not in that order).
Combinations with CodeineIt is dangerous to combine codeine with other drugs which cause respiratory depression or inhibition of neural activity, such as other opioids, alcohol, benzodiazepines, muscle relaxants, sleeping pills, or other products which may have sedative, hypnotic, or pain-relieving products. Combinations like this are synergistic, in a bad way- combined depression from two or more of the drugs will be potentiated, making them more dangerous together than they are alone. It is important to mention to a doctor if you are taking any medications, even if you don't think they will be an issue.
Different Uses for CodeineCodeine is used for treating mild to moderate pain (analgesia), coughing (antitussion), and diarrhea (antidiarrheal). It is also approved for the treatment of irritable bowel syndrome. More codeine is needed to treat pain than coughing and diarrhea.
Pharmacology of CodeineCodeine (methylmorphine) is a prodrug, which means it is inactive until it is metabolized in the body. After ingestion, it enters the digestive tract and then through the hepatic portal vein, enters the liver. There, it is converted to morphine (about 5-10% by enzyme CYP2D6), norcodeine (about 10%; CYP3A4) and codeine-6-glucuronide (about 70%; UGT2B7). Codeine-6-glucuronide is thought to be active, and primarily responsible for codeine's analgesic properties. A small population of Caucasians and Asians are poor metabolizers (less CYP2D6) of codeine, therefore it is thought that they might experience less analgesia with codeine; however, because C6G is thought to be the primary active chemical, it should be less of an issue.
C6G and morphine activate mu-opioid receptors, receptors for endogenous opioids (endorphins, etc.). G-protein coupled receptors, their activation, through a complex mechanism, increases GABA release, resulting in neuronal inhibition (ie sedation, drowsiness). Three types of mu-opioid receptors exist: m1, 2, and 3. M1 has the highest affinity for endogenous and synthetic opioids, however binding to m2 is thought to be responsible for induced euphoria. Each receptor subtype is responsible for different subjective and objective effects. Not much is known about m2, and even less about m3.
LD50 (mg/kg) :
Mice : 300 subcutaneously (hydrochloride monohydrate)
Chemistry of Codeine
Column 1 Column 2 Systematic (IUPAC) name: (5[alpha],6[alpha])-7,8-didehydro-4,5-epoxy-3-methoxy-17-methylmorphinan-6-ol Synonyms: methylmorphine, morphine monomethyl ether, morphine 3-methyl ether, Codicept; Galcodine (phosphate) Molecular Formula: C18H21NO3 Molar mass: 299.37 g/mol, 334.82 g/mol (hydrochloride), 696.81 g/mol (sulfate), 397.36 g/mol (phosphate) CAS Registry Number: 76-57-3, 1422-07-7 (hydrochloride), 1420-53-7 (sulfate), 52-28-8 (phosphate) Melting Point: 154-156°C (after drying at 80°C), ~280°C (hydrochloride dihydrate) Boiling Point: no data Flash Point: no data Solubility: Freebase : 1 g dissolves in 120 mL water, 60 mL water (80°C), 2 mL alcohol, 1.2 mL hot alcohol, 13 mL benzene, 18 mL ether, 0.5 mL chloroform; freely soluble in amyl alcohol, methanol, dilute acids; almost insoluble in petroleum ether, aqueous solutions of alkali hydroxides. Hydrochloride : 1 g dissolves in 20 mL water, 1 mL boiling water, 180 mL alcohol. Sulfate : 1 g dissolves in 30 mL water, 6.5 mL water (80°C), 1300 mL alcohol; insoluble in alcohol, ether. Phosphate : freely soluble in water, very soluble in hot water; slightly soluble in alcohol, more soluble in hot alcohol. Additionnal data: monohydrate : density 1.32, sublimes at 140-145°C under 1.5 mmHg, pK (15°C) 6.05, pH of saturated aqueous solution : 9.8. Notes: Freebase : crystallises as monohydrate, aspect : orthorhombic sphenoïdal rods or tablets (octahedra), crystallized from water or dilute alcohol. Hydrochloride : aspect small needles, some decomposition @ 280°C. Sulfate : aspect : white crystals or crystalline powder, store in airtight container, protect from light. Phosphate : fine, white, needle shaped crystals or crystalline powder, odorless, affected by light
The dangers of CodeineA significant danger of codeine use, like all other opiates/opioids, is respiratory depression. At normal recreational or prescribed levels, this is not usually an issue, however larger doses of codeine can cause difficulty breathing. This is dose-dependent, and can also be potentiated if other drugs are used which have the same side effect, such as benzodiazepines or alcohol.
Another danger of codeine, again like other opiates/opioids, is addiction. Continued use of codeine can cause physical and mental dependence. The addiction potential of codeine is much less than that of other more potent opioids such as heroin or oxycodone, as the euphoria produced tends to be less intense than that of other similar drugs. However, it is not to be underestimated. If physical dependence develops, cessation of codeine use can cause withdrawal symptoms, most commonly cravings, sweating, insomnia, nausea, diarrhea, muscle spasms, chills, pain, and irritability.
Producing CodeineCodeine is present naturally in opium from 0.7 to 2.5% , and can be extracted through various techniques. For pharmaceutical use, it is commonly synthesized from morphine through methylation, as morphine is much more abundant in opium.
Forms of CodeineCodeine is present naturally as an alkaloid in opium. It is also present in countless OTC and prescription medications, usually as codeine sulfate (one of its salts). Common preparations include Tylenol or Empirin #1, 2, 3, and 4 (8, 15, 30, and 60mg of codeine, respectively), and Nurofen Plus (12.8mg of codeine). Keep in mind that most preparations also include colors, binders, and usually few hundred milligrams of acetaminophen.
Regular users of codeine containing cough syrups have been shown to be defficient in the brains white matter which has been linked to increased impulsiveness. 
Legal status of Codeine
Controlled substance : 21 CFR 1308.12 
USCodeine is a Schedule II drug in the US, which means it is illegal to sell without a DEA license, and illegal to buy or possess without a license or prescription. (To be clear, codeine is not found in any OTC product in the US.) Products which contain less than 90mg of codeine per dosage unit are Schedule III.
EUIn the UK, codeine is a Schedule II/Class B drug. OTC products are available with small amounts of codeine.
History of CodeineCodeine has been used throughout history, as it is present in smoked opium. It was first isolated from opium in 1832 by Jean-Pierre Robiquet
ReferencesMerck Index, fifteenth edition (2013)
 American society of nueroradiology, Abnormal White Matter Integrity in Chronic Users of Codeine-Containing Cough Syrups: A Tract-Based Spatial Statistics Study