A “fourth wave” of the opioid epidemic is being driven by a mix of stimulants such as cocaine and methamphetamines with fentanyl. The mixture presents a steep challenge to efforts to reduce overdoses, because many people who use stimulants don’t know they are at risk of ingesting opioids and don’t take precautions against overdoses, NPR reports.

Using drug-checking tools such as fentanyl test strips is the only way to know if cocaine or other stimulants contain fentanyl. Federal health officials have embraced fentanyl test strips and other rapid drug-detecting tools that could help prevent overdose deaths. Fentanyl test strips cost as little as $2 per two-pack online, and can be obtained at no cost from many front-line organizations.

“Folks who are using stimulants, and not intentionally using opioids, are unprepared to respond to an opioid overdose…because they don’t perceive themselves to be at risk,’’ said Jaclyn White Hughto, a Brown University epidemiologist and a principal investigator in a new, unpublished study called Preventing Overdoses Involving Stimulants.

The researchers surveyed more than 260 people in Rhode Island and Massachusetts who use substances. More than 60% of the people interviewed in Rhode Island had bought or used stimulants that they later found out had fentanyl in them.