“If they’ve got one needle and they’re not in the program, they’re going to jail,” Sheriff McClain said.

Dr. Don Des Jarlais, the director of research for the chemical dependency institute at Mount Sinai Beth Israel hospital in New York, said the most successful needle exchange programs let participants pass out syringes to peers who remain in the shadows instead of requiring everyone to sign up. Arresting drug users who are not officially enrolled in the program “makes it hard to build trust,” Dr. Des Jarlais said, adding, “You’re not going to be able to get enough syringes out to really stop the epidemic if you have those types of restrictions.”

Local supporters of the needle exchange say a limited program is better than none, and believe that improvements will come with time. Last week, the state legislature sent a bill to Mr. Pence that would allow communities to create needle exchange programs for up to a year if they are experiencing an epidemic of H.I.V. or hepatitis C because of intravenous drug use. Mr. Pence said he would sign the measure, noting in a statement that it would allow only “limited and accountable” needle exchange programs, and only “where public health emergencies warrant such action.”

For now, the program here is giving out a maximum of 140 clean needles per user per week to whoever goes to the outreach center or accepts them from the roaming minivan. Ms. Combs said some people told her they injected as often as 15 times a day, and the exchange is erring on the side of providing slightly more than people need. She has passed out needles at a house where the owner, an older woman known as Momma, sits on the porch while a steady stream of visitors comes to shoot up inside. She has knocked on the door of a trailer where, she said, “multiple family members live and the daughters all prostitute themselves out and everyone is doing drugs.” One recent afternoon, on a street fragrant with lilacs, a young woman on a bicycle declined Ms. Combs’s offer of clean needles, saying she already had some — and H.I.V.

“I know I need the medicine to slow it down,” she murmured.

At a run-down house with a wheelchair on the porch, Tiffany Prater, 27, walked out to greet the van, saying, “The needles ain’t lasting me long enough.” She beckoned two men out of the house to get some, too.

“This little boy right here needs a card,” she told Ms. Combs, gesturing toward an expressionless friend whose eyes kept slipping shut. “You got some extra Neosporin and stuff? Because look how bad his arms is.”