Pills spilling from bottleBoth oxycodone and Opana (oxymorphone) are potent opioid agonists, used for the treatment of moderate to severe pain.


When is comes to comparing potency and "equal doses" of different opioids, most practitioners use what are known as "Morphine Milligram Equivalent Charts". These charts allow practitioners to convert between doses of opioid medications and get equal (or nearly equal) analgesic effects.


Comparing Potency Of Oxycodone To Oxymorphone

Referencing the morphine milligram equivalent charts, oxymorphone is approximately twice as potent as oxycodone. For example, the approximate "equianalgesic" dose of oxycodone 20mg is 10mg of oxymorphone.


In your case, being on oxymorphone immediate release 20 mg three times per day, the equal analgesic dose of oxycodone would be 40mg three times per day. An easy to use opioid conversion calculator can be found here: Opioid Calculator.


Other Differences

It is important to note that while conversion charts between opioids allow us to have a good idea of comparative potency, there are other factors to consider when selecting or changing opioid drugs, such as:

  • The pharmacokinetic profile of the drugs (e.g. onset of action, duration of action)
  • Which opioid receptors are affected by the drugs
  • Dosage form preference
  • Cost
  • Adverse effects


Below, we discuss a few of the big differences between oxycodone and oxymorphone.


Contraindications

Oxycodone and oxymorphone have similar adverse effects. However, oxymorphone is contraindicated in those with moderate to severe liver impairment. One metabolite of oxymorphone, normeperidine, can accumulate in patients with liver impairment, which can increase the risk of seizures.


Pharmacokinetics

Oxycodone and oxymorphone have a similar onset of action and duration of action. However, oxymorphone may work slightly faster than oxycodone as oxymorphone is more lipid soluble and crosses the blood brain barrier more readily. According to studies, oxymorphone reaches peak concentrations in 30 minutes, versus around 60 minutes for oxycodone. Oxymorphone also has a longer half-life than oxycodone.


Food greatly affects oxymorphone absorption. Taking oxymorphone with food can increase maximum concentrations, increasing the risk of adverses effects. Therefore, the prescribing information for oxymorphone recommends the following:

As a result [of studies regarding food intake], OPANA [oxymorphone] should be dosed at least one hour prior to or two hours after eating.
Oxycodone may be taken with or without food.


Lastly, oxymorphone is not thought to be extensively metabolized by the CYP family of metabolizing enzymes and therefore, may have fewer drug interactions.


Mechanism Of Action

Oxycodone and oxymorphone are opioid agonists. However, they have slightly different mechanisms of action.


There are 3 main opioid receptors, µ (mu), kappa and delta. The µ is the main site of action for both oxycodone and oxymorphone, which produces the analgesic effects of the drugs. However, oxymorphone also acts as a weak agonist at the delta opioid receptor, which most likely enhances the pain relieving ability of the drug.


Oxycodone is thought to have stronger effects on the kappa receptors in the spinal cord.


Summary Of Differences

  • Opana (oxymorphone) is approximately twice as potent as oxycodone on a mg to mg basis.
  • Oxymorphone reaches peak concentrations faster than oxycocone.
  • Oxymorphone has a longer half-life than oxycodone, and therefore takes longer to eliminate from the body.
  • Both oxycodone and oxymorphone are potent µ (mu) agonists. However, oxycodone has stronger effects on kappa receptors while oxymorphone has stronger effects on delta receptors.
  • Oxymorphone is contraindicated in those with liver disease due to the potential toxic accumulation of metabolites.
  • Oxymorphone should be taken on an empty stomach as food greatly affects drug absorption.
  • Oxymorphone is thought to have less drug interactions than oxycodone since it is not metabolized by CYP enzymes.