Oxycodone is a narcotic medication used for the treatment of chronic pain. It is reserved for those patients who have tried other nonnarcotic pain relievers without success. Most often, other treatment such as physical therapy is tried first as well.
Doctors know that patients don't all respond to the same drug in the same way. Most often this is because people metabolize (break down and use) drugs at different rates. Since the same amount of drug is usually prescribed for everyone, half the people get too much drug. The other half doesn’t get enough.
Research has shown that most of the medications like oxycodone used for pain control are metabolized by an enzyme called CYP2D6. Genetic variations of that enzyme cause people to metabolize drugs at different rates.
A small percentage are ultrarapid metabolizers. The drug is broken down and passed out of the body too quickly to do much good. Your sister may fall into this category. Others break it down slowly enough that a steady amount of the drug stays in the body longer. This may describe your situation.
Choosing the right drug for each patient and prescribing the best dose is part of pain management. If she is not getting the pain control she needs, then it's time to get back with the doctor or pain management team for a re-evaluation.