List of products in the National Drug Code with proprietary name . This medication is used to help relieve sudden (breakthrough) pain in people who are already taking opioid medication around-the-clock for cancer pain. Fentanyl belongs to a class of drugs known as opioid analgesics. It works in the brain to change how your body feels and responds to pain. This medication should not be used to relieve mild or short-term pain (such as due to headache/migraine, dental procedures, surgery).