However, there’s evidence that a two-inch fentanyl test strip can help drug users avoid overdosing. When dipped into the drug, the strip reveals—with the presence of a red line, or not—whether a drug contains fentanyl. Researchers suspect that if more drug users had access to these strips, they could test their drugs before using and use less, or possibly not even at all.

For a new study in the International Journal of Drug Policy, researchers from RTI International and the University of California, San Francisco, studied 125 heroin users in Greensboro, North Carolina to see if the test strips, which were distributed through a local needle-exchange program, would change the way the individuals used their drugs. Through an online survey, 81 percent of the drug users reported using the strips, and 63 percent got a positive result for fentanyl. However, those who did see the positive result were five times more likely to change the way they used in an attempt to avoid overdosing. They might have used less of the drug than usual, for example, or snorted it instead of injecting it, which results in less of the drug absorbing into the bloodstream.

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