What is oxycodone? Oxycodone is a prescription opioid pain medication used to treat moderate to severe pain, such as pain after surgery or from serious injuries or illnesses. It works by changing how the brain and nervous system respond to pain. Because it can cause dependence and addiction, oxycodone is classified in the U.S. as a Schedule II controlled substance, meaning its use is strictly regulated by law. Common side effects include drowsiness, constipation, nausea, and slowed breathing, and misuse can lead to overdose or death.