However, despite the greater in vitro activity of some of its metabolites, it has been determined that oxycodone itself is responsible for 83.0% and 94.8% of its analgesic effect following oral and intravenous administration, respectively. Oxymorphone plays only a minor role, being responsible for 15.8% and 4.5% of the analgesic effect of oxycodone after oral and intravenous administration, respectively. Although the CYP2D6 genotype and the route of administration result in differential rates of oxymorphone formation, the unchanged parent compound remains the major contributor to the overall analgesic effect of oxycodone. In contrast to oxycodone and oxymorphone, noroxycodone and noroxymorphone, while also potent MOR agonists, poorly cross the blood–brain barrier into the central nervous system, and for this reason are only minimally analgesic in comparison. κ-opioid receptor
In 1997, a group of Australian researchers proposed (based on a study in rats) that oxycodone acts on KORs, unlike morphine, which acts upon MORs. Further research by this group indicated the drug appears to be a high-affinity κ2b-opioid receptor agonist. However, this conclusion has been disputed, primarily on the basis that oxycodone produces effects that are typical of MOR agonists. In 2006, research by a Japanese group suggested the effect of oxycodone is mediated by different receptors in different situations. Specifically in diabetic mice, the KOR appears to be involved in the antinociceptive effects of oxycodone, while in nondiabetic mice, the μ1-opioid receptor seems to be primarily responsible for these effects. Pharmacokinetics
Instant-release absorption profile
Oxycodone can be administered orally, intranasally, via intravenous, intramuscular, or subcutaneous injection, or rectally.
Oxycodone in combination with naloxone in managed-release tablets, has been formulated to both deter abuse and reduce "opioid-induced constipation". Dependence and withdrawal
The risk of experiencing severe withdrawal symptoms is high if a patient has become physically dependent and discontinues oxycodone abruptly. Medically, when the drug has been taken regularly over an extended period, it is withdrawn gradually rather than abruptly. People who regularly use oxycodone recreationally or at higher than prescribed doses are at even higher risk of severe withdrawal symptoms. The symptoms of oxycodone withdrawal, as with other opioids, may include "anxiety, panic attack, nausea, insomnia, muscle pain, muscle weakness, fevers, and other flu-like symptoms".Withdrawal symptoms have also been reported in newborns whose mothers had been either injecting or orally taking oxycodone during pregnancy. Hormone levels
As with other opioids, chronic use of oxycodone (particularly with higher doses) often causes concurrent hypogonadism (low sex hormone levels). Overdose
In high doses, overdoses, or in some persons not tolerant to opioids, oxycodone can cause shallow breathing, slowed heart rate, cold/clammy skin, pauses in breathing, low blood pressure, constricted pupils, circulatory collapse, respiratory arrest, and death.In 2011, it was the leading cause of drug-related deaths in the U.S. However, from 2012 onwards, heroin and fentanyl have become more common causes of drug-related deaths.Oxycodone overdose has also been described to cause spinal cord infarction in high doses and ischemic damage to the brain, due to prolonged hypoxia from suppressed breathing. Interactions
Oxycodone is metabolized by the enzymes CYP3A4 and CYP2D6.