These alkaloids are divided into two distinct chemical classes, phenanthrenes and isoquinolines. It is one of the most effective drugs known for the relief of pain, and remains the standard against which new analgesics are measured. It may be administered orally, subcutaneously, intramuscularly, and intravenously, the latter method being the one most frequently used by addicts. Tolerance and physical dependence develop rapidly in the user. This alkaloid is found in opium in concentrations ranging from 0.7 to 2.5 percent.
Most codeine used in the United States is produced from morphine.
Compared to morphine, codeine produces less analgesia, sedation and respiratory depression, and is frequently taken orally. Codeine is medically prescribed for the relief of moderate pain. Codeine is an effective cough suppressant and is found in a number of liquid preparations. Codeine products are also used to a lesser extent as an injectable solution for the treatment of pain. It is by far the most widely used naturally occurring narcotic in medical treatment in the world.