Fentanyl is used to treat severe pain in cancer patients. It is used for breakthrough cancer pain, which are flares of pain that “break through” after a routine pain medicine has been used. Fentanyl belongs to the group of medicines called narcotic analgesics. It is only used in patients who are already taking narcotic analgesics. Fentanyl acts in the central nervous system (CNS) to relieve pain. Some of its side effects are also caused by actions in the CNS. When a narcotic is used for a long time, it may become habit-forming or cause mental or physical dependence. However, people who have continuous pain should not let the fear of dependence keep them from using narcotics to relieve pain. Mental dependence (addiction) is not likely to occur when narcotics are used for this purpose. Physical dependence may lead to withdrawal side effects if treatment is stopped suddenly. Withdrawal side effects can usually be prevented by gradually reducing the dose over a period of time before the medicine is stopped completely. Talk to your doctor about the benefits of this medicine and how to prevent withdrawal side effects.