What does fentanyl do other than kill, and why would anyone put it in drugs they are selling?
It’s an opiate like heroin, morphine, oxycodone. Similar effects. It’s ideal for drug smuggling as it’s very potent, 50-100x as much as heroin. 1gram of fentanyl could probably make 3,000-10,000 doses, depending on desired strength. So you can traffic considerably smaller parcels and still bring the same number of doses to market.
Given it’s low cost & completely synthetic nature (heroin is still derived from poppy latex), it’s a natural choice for anyone looking to counterfeit opioid pills like Percocet or oxycodone. It doesn’t seem logical to cut cocaine or other stimulant drugs with it but I assume that these folks just had the fentanyl lying around and decided to include it for the hell of it.
As to why the rise of counterfeit pills, well, updated FDA prescribing guidelines have sharply decreased the supply of legitimate pills available. Gotta meet the demand somehow …