This is one of the reasons I hugely support legalized drug testing, and having law enforcement keep out of the way of drug testing orgs, because it's a public health risk.
Reagent testing kits are pretty easy for the individual consumer to buy (legally) and test their substances with before use.
In the UK, the charity The Loop is making great strides with testing and harm-reduction awareness, they've been running stalls at large festivals for a few years now and have a few permanent centers in large cities. When they discover drugs being sold with dangerous adulterants, they make it known on social media, and relay the info to paramedics and welfare. Has definitely saved a few lives!