Before he started taking methadone, Vinny Parisi had overdosed 16 times from using street drugs, including fentanyl. Eating out of garbage cans and sleeping under a bridge in Harlem, he finally hit bottom, he said.

Now, Mr. Parisi goes every weekday morning to an R.V.-size white van parked at a Days Inn in the South Bronx. Within a few minutes, he drinks a bright pink fluid — a dose of methadone — saving him the hours of commuting and waiting it often takes to visit a brick-and-mortar clinic to get the drug.

“This definitely works, I’m living proof,” Mr. Parisi said on a recent Tuesday outside the van, where he was waiting with about a dozen other men from his residential drug treatment program. He is only 30 years old, but has been in and out of treatment programs since age 15, after starting to abuse pain pills on Staten Island.

“My mother sent me a picture of me and 12 friends, and I’m the only one left alive,” he said.

Mr. Parisi is one of an estimated 450,000 Americans who take methadone, a powerful weapon in the fight against the fentanyl overdose crisis hiding in plain sight. Methadone, itself a potent opioid, has been used for decades to treat people addicted to drugs like heroin. But it can also be hard to come by, because of government rules that have kept its distribution tightly controlled.

As America’s deadly overdose crisis has worsened, however, some of those rules are loosening. Now, some public health experts hope that mobile treatment programs, like the one in the Bronx, will help increase access.