Subsys (generic: fentanyl) is a prescription medication used to treat breakthrough pain (sudden temporary flares of pain) in adults with cancer who are already taking other opioid pain medicines around-the-clock for cancer pain.  This medicine belongs to a group of drugs called opioid narcotic analgesics which help to relieve pain by binding to certain opioid receptors in the body. 

This medication comes in liquid form to be sprayed under the tongue every 4 hours as needed.

Common side effects of Subsys include nausea, vomiting, constipation, and drowsiness. Do not drive or operate heavy machinery until you know how this medication affects you.