As a 
physician prescribing “pain killers” for chronic painful conditions, you know that obtaining random urine drug screens (UDS) is essential for assuring compliance of the patient to agreed upon treatment plan.
Knowing the metabolism of common opioids (opiates) helps you understand why patients can test positive on a urine drug screen (UDS) for drugs you are not actually prescribing; when sent for gas chromatography (or mass spectrometry) analysis.
For example, your patient being prescribed Percocet (oxycodone) will test positive for Opana (oxymorphone).
Below is an image of a chart I made to show the basic metabolism of opioids (click to enlarge).
Download a PDF of the chart HERE.
 helps you understand why patients can test positive on a urine drug screen (UDS) for drugs you are not actually prescribing.){kind=link}
Thank you. I have been asked by many fellow pain patients why they have been told that they had odd test results.