As explained here,

There is a new tool to help battle the opioid epidemic that works like a pregnancy test to detect fentanyl, the potent substance behind the escalating number of deaths roiling communities around the country.

The test strip, originally designed for the medical profession to test urine, can also be used off-label by heroin and cocaine users who fear their drugs have been adulterated with the synthetic opioid fentanyl. The strips are dipped in water containing a minute amount of a drug and generally provide a result within a minute—with one line indicating positive for fentanyl, and two lines negative.

Overdose-prevention organizations in the U.S. first started buying and handing out fentanyl test strips about two years ago, and they caught on quickly. Now, states like California and Rhode Island and cities such as Baltimore, Philadelphia and Columbus, Ohio, are distributing them, or plan to soon.

The moves have encountered opposition. Elinore McCance-Katz, head of the federal government’s Substance Abuse and Mental Health Services Administration, said the approach relied on the flawed premise that drug users would make rational choices.

diva) we have to outlaw drugs because people are not rational enough to use them safely;

b) if prohibition makes it difficult for users to determine potency and quality, that is unfortunate;

c) but if users respond to this uncertainty by taking steps that reduce the risks, we cannot trust them to do that since they might not get it exactly right.