By Anthony McDaniel, M.D.
Fentanyl (also known as fentanil, brand names Sublimaze, Actiq, Durogesic, Duragesic, Fentora, Onsolis, Instanyl, Abstral, and others) is a potent synthetic narcotic analgesic with a rapid onset and short duration of action. It is a strong agonist at the μ-opioid receptors. Historically it has been used to treat chronic breakthrough pain and is commonly used before procedures as an anesthetic in combination with a benzodiazepine. Fentanyl is approximately 100 times more potent than morphine, with 100 micrograms of Fentanyl approximately equivalent to 10 mg of morphine and 75 mg of pethidine (meperidine) in analgesic activity. It has an LD50 of 3.1 milligrams per kilogram in rats, and an LD50 of 0.03 milligrams per kilogram in monkeys. Fentanyl was first synthesized by Dr. Paul Janssen in 1960 following the medical inception of pethidine several years earlier. Janssen developed fentanyl by assaying analogues of the structurally-related drug pethidine for opioid activity. The widespread use of fentanyl triggered the production of fentanyl citrate (the salt formed by combining fentanyl and citric acid in a 1:1 stoichiometry), which entered the clinical practice as a general anaesthetic under the trade name Sublimaze in the 1960s. Following this, many other fentanyl analogues were developed and introduced into the medical practice, including sufentanil, alfentanil, remifentanil, and lofentanil.
In the mid 1990s, fentanyl saw its first widespread palliative use with the clinical introduction of the Duragesic patch, followed in the next decade by the introduction of the first quick-acting prescription formations of fentanyl for personal use, the Actiq lollipop and Fentora buccal tablets. Through the delivery method of transdermal patches, fentanyl is currently the most widely used synthetic opioid in clinical practice, with several new delivery methods currently in development. Fentanyl's major side effects (more than 10% of patients) include diarrhea, nausea, constipation, dry mouth, somnolence, confusion, asthenia (weakness), and sweating and, less frequently (3 to 10% of patients), abdominal pain, headache, fatigue, anorexia and weight loss, dizziness, nervousness, hallucinations, anxiety, depression, flu-like symptoms, dyspepsia (indigestion), dyspnea (shortness of breath), hypoventilation, apnea, and urinary retention. Fentanyl use has also been associated with aphasia. Despite being a more potent analgesic, fentanyl tends to induce less nausea, as well as less histamine-mediated itching, in relation to morphine.
The fentanyl patch has been associated with altered mental state leading to aggression in an anecdotal case report. Like other lipid-soluble drugs, the pharmacodynamics of fentanyl are poorly understood. The manufacturers acknowledge there is no data on the pharmacodynamics of fentanyl in elderly, cachectic or debilitated patients, frequently the type of patient for whom transdermal fentanyl is being used. This may explain the increasing number of reports of respiratory depression events since the late 1970s. In 2006 the U.S. Food and Drug Administration began investigating several respiratory deaths, but doctors in the United Kingdom had to wait until September 2008 before being warned of the risks with fentanyl.
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