That s not correct at all.
It is 2 mg, not 2 ng.
A nano gram is 1/1000th of a microgram. And a microgram is 1/1000th of a milligram.
Fentanyl patches range from 12 to 100 micrograms.
If 2 nano grams would kill you, the patches would kill everyone who is prescribed them.
concentration in blood toxicology (ng/ml), not dosage
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3576505/