An earlier study by Johns Hopkins University also suggested that most injecting-drug users were interested in knowing whether their drugs contained fentanyl before using, and that they would modify their behavior if they knew.

Jon Zibbell, a public-health analyst at RTI International and an author of the study, says his paper helps build the evidence base that might lead to more funding for the test strips, even as more studies continue to be conducted. Health officials often cite a lack of scientific evidence as a reason not to implement a controversial public-health intervention.

“Harm reduction at its core is a scrappy self-made movement,” says Daniel Ciccarone, a UCSF professor who co-authored the study. “Syringe exchange and naloxone peer distribution came out of this movement and have gone mainstream. But the [test strips] need an evidence base in order to become the next intervention in this legacy.”

Fentanyl is deadlier than heroin and prescribed opioids.

A few cities—including Baltimore; Philadelphia; Columbus, Ohio; and Burlington, Vermont—have started providing the test strips alongside clean needles. The California public-health department pays for the distribution of strips through needle exchanges.

But one hurdle to broader distribution is paraphernalia laws that prohibit the use of devices to aid in doing drugs. This restriction has been loosened when it comes to clean syringes, but it might still be applied to test strips. (The District of Columbia and Maryland recently exempted the strips from their definition of paraphernalia.) Zibbell predicts that test strips will start to be introduced into cities that already have progressive drug policies, such as those with long-standing needle exchanges, and will eventually flow to areas that have been more resistant to harm-reduction programs. Fifteen states still outlaw needle exchanges, for example, and Charleston, West Virginia, at the epicenter of the opioid epidemic, recently closed its needle-exchange program.

Another obstacle is the $1 cost of the strips, which can quickly balloon, since the average heroin user injects four times a day. Until the strips are more widely available, some have suggested that drug users should simply assume that all their heroin contains fentanyl, and act accordingly.

But, according to Zibbell, “that’s like saying, ‘Assume everyone you have sex with has chlamydia.’” Most people simply don’t take precautions unless the danger they face is real.

Zibbell hopes that the test strips are just a start, and that eventually needle exchanges and other sites can offer other drug-testing tools, such as spectrometers.

“We have a poisoning epidemic,” Zibbell said. “When there’s E. coli in the lettuce, you test the lettuce. You have to test the product to see what’s in it.”