Fentanyl is an opioid analgesic drug. The transdermal delivery system is intended to allow the passive diffusion of the medication over a long period of time while maintaining a constant therapeutic dose. Transdermal fentanyl has been evaluated for the management of chronic cancer-related pain. Transdermal fentanyl also has been examined in relation to opioids by other routes for its effect on opioid-related constipation. The use of transdermal fentanyl in patients who are not opioid tolerant and for acute pain is contraindicated due to a risk of life-threatening hypoventilation.