Really the duration can vary quite a lot. I was put under on fentanyl for having my wisdom teeth taken out and woke up in the middle of the operation in extreme pain as someone was digging my teeth out with a metal instrument. I moaned at him, through a mouth stuffed with gauze pads, to get the fuck off me and let me out of there if he wasn't giving me more meds. He proceeded to angrily accuse me of taking dope, which I had, quite a few times, but never that much, in high school a few years before. I had already explained this before the procedure and assumed they had adjusted the dose. Explaining this again, with tears of pain rolling down my face, the nurse told me it's okay and, I assume out of some attempt to comfort me, that she'd taken dope too. They gave me the fentanyl and I passed out again. My opiate use in high school was infrequent and I'd never experienced withdrawals in the slightest, though I'd say I probably took a double or triple dose a few times, mostly because it never seemed to do that much for me. I was probably 16 when I started my experimentation and it ended by the time I was 17. Freshman year of college I was suffering stomach pain and headaches, so the doc at school prescribed Reglan and a small amount of Vicodin. Painkillers for a headache? By the end of the next day I had taken them all, in total 25mg Hydrocodone spread out over a day and a half. I take ibuprofen for headaches anyway. That was probably 8-12 months before my wisdom teeth procedure, but somehow either that one time recently or the times I took opiates in high school years before were enough for the fentanyl to wear off and for me to get yelled at by a doctor for it. I can't figure out if I always had a high tolerance or if opiate tolerance never goes away. I'm looking at this chart of equivalent potency https://en.m.wikipedia.org/wiki/Equianalgesic and it seems like I probably took the equivalent of a 15 mg oxycontin at the highest dosage I ever took, 22.5mg Hydrocodone, so it's really a mystery to me why I would have an effect like that. In my opinion, the problem here is that a higher dose was recommended instead of a longer acting opioid. However, I think people should always research their medication and talk to their pharmacist. I never took the Reglan because there's a risk of Parkinsons. My friend was prescribed a huge dose of antidepressants and ended up in the hospital with serotonin syndrome. Doctor knows best is a lie when it comes to medication. Pharmacist knows best. Always ask the pharmacist.