I'm wondering how long would it take for the symptoms of a fentanyl overdose to become evident? How long do the effects of fentanyl last? Thank you in advance for your answers, I recently lost a loved one o a fentanyl overdose, and I'm trying to find answers. He had used fentanyl about 8 hours earlier, and was found still alive at the time (unfortunately, he didn't make it), and I'm wondering if that timeline seems a little off...