Proponents say the ability to check drugs for the presence of lethal fentanyl may save lives. But critics say the strips enable drug use.“You smoke weed?” Eufamia Lopez asked the half-dozen young men lounging on benches in a public housing courtyard in the South Bronx.The soft September air reeked of the obvious answer.Ms. Lopez, who works for a New York University health support program, plunged into her spiel. Street drugs — meth, coke, molly, Xanax, heroin and even marijuana — are being cut with fentanyl these days, she said, which can kill you. But you can test your supply before using it to see whether there’s any fentanyl in it. She was giving out free kits.She had their attention, and not just because she is 5-foot-10, frank but ebullient, with cascading black curls scarcely held in check by a brightly colored scarf. The neighborhood, Mott Haven, has one of New York City’s highest overdose rates. After she recited the simple instructions, each man readily accepted a kit with three fentanyl test strips. One asked for a second kit.“I appreciate you,” she said, rewarding him with a smile.The spread of fentanyl, a cheap synthetic opioid 50 times as lethal as heroin, into most kinds of illicit drugs has pushed fatal overdoses to record highs in the United States. Fentanyl test strips have become a popular but contentious tool in response. Supporters say they help drug users make lifesaving decisions. Opponents contend that they facilitate drug use.Test strips are a part of a broader approach called harm reduction, which holds that ending the overdose crisis can be achieved only by first ameliorating the deadliest risks of drug use, then taking steps to curb behavior, such as addiction treatment. President Biden is the first president to embrace harm reduction, and he has made fentanyl test strips a key component in his proposed $307 million harm-reduction drug-control strategy. Within the past year, about 10 states — including Louisiana, Tennessee, Alabama and Georgia, where hard-line abstinence views are more typically favored — have legalized test strips and made them more available.But test strips are illegal in about 20 states — including Florida, Texas, Kansas and Kentucky — classified as “drug paraphernalia.”Critics say test strips encourage drug use by giving users the green light if the supply is free of fentanyl. To some opponents, the test strips are even more objectionable than other forms of harm reduction, like distributions of clean syringes, because those at least prevent the spread of H.I.V., hepatitis and other dangerous infections to users and nonusers.A needle on the ground in Mott Haven. “Before Covid I don’t remember people feeling comfortable enough to shoot up in front of everybody,” Ms. Lopez said. “The world has definitely changed for poor people.”Instructions on how to use the strips. In May, Kansas legislators blocked a proposal to legalize the strips. “Fentanyl strips don’t save lives. Let’s be clear. There are individuals that want fentanyl in the drug that they’ve purchased or acquired,” Molly Baumgardner, a Republican state senator, said at the time, according to the Kansas Reflector, which reports on state government.The Biden administration’s drug czar refutes such criticism. “There is no scientific evidence to support this notion that harm-reduction services like fentanyl test strips somehow encourage drug use, but there is significant evidence to support the fact that these tools can save lives,” said Dr. Rahul Gupta, director of the federal Office of National Drug Control Policy.Fentanyl Overdoses: What to KnowCard 1 of 5Devastating losses.