Ok, first of all, I'd like to know what kind of DENTAL operation would have them consider giving you an Rx for Fentanyl. Fentanyl is used either as an anesthetic DURING the surgery or as an adjunct to ongoing opiod therapy. I'm fairly positive your facts are crooked and the fentanyl is for during the procedure, not after. Anyway, as far as the question goes, take the Demerol, hands down. The fentanyl high lasts all of a half hour if you're lucky. Demerol lasts much longer, feels similar enough. Fentanyl is not all that unless delivered in a constant strong stream.