Fentanyl alone can cause sudden death due to cardiac arrest or severe anaphylactic reaction.
Blood concentrations of approximately 7 ng/ml or greater have been associated with fatalities where poly-substance use was involved.
George Floyd had 11 ng/mL of Fentanyl & 19 ng/mL of Meth, plus other substances in his blood at the time of death.
https://www.hennepin.us/-/media/hennepinus/residents/public-safety/documents/Autopsy_2020-3700_Floyd.pdf
https://www.emcdda.europa.eu/publications/drug-profiles/fentanyl_en
CDC reports that, on June 23, 2016, twelve people were taken to emergency rooms in New Haven, CT with symptoms of opioid overdose.
Patient E had a blood concentration of 11 ng/mL (same as George Floyd's Fentanyl in his blood) and was DEAD UPON ARRIVAL….
Despite patient given the Fentanyl antidote naloxone…
The patients who were dead on arrival had gone into cardiac arrest due to blood concentrations of fentanyl
https://www.acsh.org/news/2017/02/02/fentanyl-overdose-dont-count-naloxone-save-you-10822?fbclid=IwAR20KOn0sk8WtIUO2HkVKYpsoAobf1q5OqaYJ9UGVnV912WWXOCnHv3B-bk