Fentanyl , which is chemically related to meperidine, has 100-fold the analgesic potency of morphine and is used in anesthesia. The drug is highly lipophilic and has a rapid onset and short duration of action (15–30 minutes). It is usually administered IV, epidurally, or intrathecally. Epidural fentanyl is used to induce anesthesia and for analgesia postoperatively and during labor. An oral transmucosal preparation and a transdermal patch are also available. The transmucosal preparation is used in the treatment of cancer patients with breakthrough pain who are tolerant to opioids. The transdermal patch must be used with caution, because death resulting from hypoventilation has been known to occur. [Note: The transdermal patch creates a reservoir of the drug in the skin. Hence, the onset is delayed 12 hours, and the offset is prolonged.] Fentanyl is metabolized to inactive metabolites by the CYP450 3A4 system, and drugs that inhibit this isozyme can potentiate the effect of fentanyl. Most of the drug and metabolites are eliminated through the urine.