Both of those illicit fentanyl and carfentanil are particularly perilous opioids that will cause a quick overdose and death when abused, but carfentanil is a lot more powerful than fentanyl. Numerous doses of the anti-overdose drug naloxone (Narcan) will not be effective to reverse an overdose. Continue examiningNasal Spray: Keep at up to 25C; usua