Codeine is a narcotic analgesic used for treating mild to severe pain, and is produced from poppy flowers, the very same flowers which also are used to make heroin and opium.
Pharmacokinetics describes the behaviour of codeine and states that it is readily absorbed from the gastrointestinal tract. It is rapidly distributed from the intravascular spaces to the various body tissues, with preferential uptake by parenchymatous organs such as the liver, spleen and kidney.
In most humans 10% of a codeine dose is transformed to morphine through demethylation in the liver. This explains the analgesic effect that is absent in individuals with the respective genetic traits ("slow debrisoquine metabolism").