Lethal Dose of Heroin vs. Lethal Dose of Fentanyl
Fentanyl is 50 times stronger than Heroin. It, like Heroin, suppresses breathing. When they are combined, they are deadly. In the following episode of Drugs, Inc. from Nat. Geo, one dealer is shown spiking a random pack of heroin in his supply in intentionally cause an overdose, which, perversely enough, causes a greater demand for his product because it is so strong.
Fentanyl is normally administered in a skin patch for intense pain.