Fentanyl is used to treat breakthrough pain (sudden episodes of pain that occur despite round the clock treatment with pain medication) in cancer patients at least 18 years of age (or at least 16 years of age if using Actiq brand lozenges) who are taking regularly scheduled doses of another narcotic (opiate) pain medication, and who are tolerant (used to the effects of the medication) to narcotic pain medications. Fentanyl is in a class of medications called narcotic (opiate) analgesics.