Death by fentanyl is a form of suffocation. It shuts down your neuro and respiratory system and you can die in minutes or even seconds. Fentanyl causes respiratory depression called "wooden chest syndrome".
The onset of poisoning caused by fentanyl can occur at a much quicker rate (sometimes within seconds) than heroin. In the majority of cases, fentanyl poisoning appears very similar to other opioid overdoses. However, atypical symptoms due to fentanyl have been reported, including:
Immediate blue or grey lips
Body stiffening/seizure-like activity
Foaming at the mouth
Confusion before becoming unresponsive