The development of the passive transdermal therapeutic system (TTS) as an effective method of drug delivery has allowed the potent, lipid soluble opioid fentanyl to be administered transdermally as an analgesic agent to patients with postoperative and chronic pain.Due to the passive nature of the TTS, drug absorption is prolonged and the development of therapeutic plasma concentrations takes up to 16 to 24 hours. Steady-state plasma concentrations of fentanyl develop readily, but cannot be adjusted rapidly to compensate for acute increases or decreases in pain after surgery. This necessitates supplementation of TTS fentanyl with small quantities of systemic opioids. The difficulty in titrating TTS fentanyl for acute pain and the associated relatively high incidence of toxicity (respiratory depression) make the system unsuitable for pain after surgery.In patients with chronic pain (especially of a neoplastic origin), TTS fentanyl has proved reasonably effective. Several noncontrolled observational studies have shown that TTS fentanyl may be as effective as oral morphine as an analgesic for cancer pain, with claims of improved quality of life and increased acceptance by patients due to the ease of use. There are few data, however, from properly controlled clinical trials to substantiate these claims at the present time. In these opioid-tolerant patients, respiratory depression has not proved to be a problem if TTS fentanyl is used correctly.