Oxycodone and codeine are both medications used for pain relief that belong to the family of drugs known as opioid analgesics. Both drugs share many structural and functional similarities, but they also differ in key ways, which cause them to have related, but non-identical, medical uses. Strength, pharmacology, and bioavailability are all points of divergence between these drugs that contribute to their different uses.
Strength in terms of potency and dosage is probably the greatest area of difference between oxycodone and codeine. Oxycodone is capable of diminishing pain to a far greater degree than codeine, and it requires only about one-tenth of the dose of codeine to provide the same amount of pain relief. Therefore, oxycodone is much better suited to control moderate to severe pain. Codeine's weaker efficacy may be desirable at times, for example, to treat mild pain, or to treat pain in individuals with little to no opioid tolerance.
The human body processes oxycodone and codeine somewhat differently, meaning that the two drugs possess different pharmacologies. After ingestion, oxycodone is metabolized and excreted within a few hours. Codeine is not very active on its own, however, and must be processed by the liver into active compounds like morphine. The need for processing by the liver means that codeine must be administered orally. Any other routes of administration bypass the liver, and therefore would leave the unconverted codeine unable to affect the brain.
Bioavailability is a term that describes how effectively a substance gets into the bloodstream from a given route of administration. Both oxycodone and codeine have relatively high maximum bioavailabilities when taken orally, but codeine's varies much more wildly. For some individuals, codeine is scarcely absorbed into the blood at all. Most people absorb oxycodone at roughly the same rate, making it a more reliable medication to use in the treatment of chronic pain.
Many drugs, including oxycodone and codeine, are affected by certain compounds in grapefruit juice, which can change the way that medications work in the human body. Liver enzymes inhibited by grapefruit juice normally break down oxycodone, so combining the two can result in this drug producing a stronger effect than it normally would, and for a longer period of time. These same enzymes convert codeine into morphine and other opioids that allow codeine to exert its effects. As a result, mixing grapefruit juice with this drug can cause a reduction in its strength and duration.