Have you tried googling it?? The DEA and RCMP estimate it to be at 2mg.
Of course if your tolerance is high it would take a larger dose.
But if you've never taken fentanyl before, and some unscrupulous drug dealer mixes it with a batch of cocaine, you would probably drop dead at 2 to 3mg.
This has happened before. A lawyer (who I knew) here in Toronto OD'ed because he thought he was just taking cocaine:
A promising Toronto criminal lawyer has become one of the city’s latest victims in the growing opioid crisis.
torontosun.com
He was never into fentanyl.
He was just a weekend coke user.
And now he's dead.
RIP