This a predictable consequence of heroin's illegal status. It's a shift by vendors to higher value density materials. During U.S. prohibition, vendors replace

This a predictable consequence of heroin's illegal status. It's a shift by vendors to higher value density materials. During U.S. prohibition, vendors replaced illegal beer with gin, moonshine, etc. (higher alcohol/mass and alcohol/volume = smaller shipment volume and probability of detection). Later, weed fell in favor to hash for similar reasons. Now we have heroin being potentiated with fentanyl and its derivatives, those being much more powerful, reducing shipment volumes and losses due to detection. In the U.S., when prohibition was reversed and booze was relegalized, low alcohol beverages returned to the market. Not an expert, but it appears to me that legalization is required for reduction of overdose rates. It may not be enough by itself, but in its absence, lasting improvement doesn't happen.