A powerful painkiller skin patch, used mostly by cancer patients, has been issued the second warning since 2005 on Friday. The United States Food and Drug Administration (FDA) has found that despite their previous warning, reports of deaths and life-threatening side effects have still come in.

Fentanyl is an opioid analgesic, first synthesized by Janssen Pharmaceutica (Belgium) in the late 1950s. However, the pharmacodynamics of fentanyl are still poorly understood and there is no accurate data on the pharmacodynamics in the elderly, cachectic or debilitated patients. Unfortunately, these are frequently the types of patients for which transdermal fentanyl is being used.

"There is an unmet need to provide patients suffering from chronic pain with safe and effective products that will not only alleviate their pain, but that will also be tolerable when used chronically," said Bob Rappaport, M.D., FDA's Director, Division of Anesthesia, Analgesia and Rheumatology Products, at FDA.

"While these products fill an important need, improper use and misuse can be life-threatening. Therefore, it is crucial that doctors prescribe these products appropriately and that patients use them correctly," he said.

In its Public Health Advisory and Health Care Professional Sheet, the FDA reminded that Fentanyl patches should only be used for opioid tolerant patients. This is possible because has a potency approximately eighty times that of morphine.

Also, the FDA said that patients and health care professionals should be aware of Fentanyl overdose symptoms, and patients need to be fully explained how often to apply the patch, how to reapply a patch that came off, how to replace a patch, and how to get rid of a patch.

Transdermal fentanyl patches are also sold on the black market as the effects of the fentanyls are similar to those of heroin, with a noticeably less euphoric 'high'. However, non-medical use of fentanyl by individuals without opiate tolerance is very dangerous and has resulted in numerous deaths.