Enough to kill every person on Earth several times.
Car-Fentanyl is widely considered a chem warfare agent.
This is GD war.
25 metric tons = 25 trillion micrograms (mcg), ~3200 mcg for each of 7.7 billion humans.
I saw 2000 - 3000 mcg cited as a lethal dose for an average size adult. The maximum recommended dose I found was under "Adjunct to general anesthesia (rarely used)" with 20-50 mcg/kg/dose IV. If the 7.7 billion averaged 65 kg this would be 50 mcg/kg each. That source also said "limit total fentanyl doses to 10-15 mcg/kg for fast tracking and early extubation." Sounds more like this is enough to kill everyone once, although you might survive your dose if you could have adequate respirator and ICU support. Kind of like after the Beslan 'rescue.'