After 11 years as an addict, Mr. Kinzly cleaned up, began working with needle exchange programs and became a research associate at the Yale School of Public Health. Then came the relapse and the overdose that nearly killed him.

"We were watching TV - I think it was the Red Sox beating the Yankees," Mr. Kinzly, 47, recalled of the evening in 2005 when he passed out in a colleague's apartment. "Because of our work he knew what to do. He dialed 911 and then injected the naloxone."

Taken in high enough doses, heroin and other opioids suppress the brain's regulation of breathing and other life-sustaining functions. Naloxone is a chemical that blocks the brain-cell receptors otherwise activated by heroin, acting in minutes to restore normal breathing.

Since its approval by the Food and Drug Administration in 1971, naloxone has become a standard treatment for overdoses, used almost exclusively by emergency medical workers. But it has lately become a tool for state and cities struggling to reduce stubbornly high death rates among opiate users. By distributing the drug and syringes to addicts and training them and their partners in preventing, recognizing and treating overdoses, the programs take credit for reversing more than 1,000 overdoses.

"From a public health perspective, it's a no-brainer," said Dan O'Connell, director of the H.I.V. prevention division in the New York State Health Department, which supports 20 naloxone programs, all but one in New York City. "For someone who is experiencing an overdose, naloxone can be the difference between life and death."

But federal drug officials say distributing naloxone directly to addicts may do more harm than good.

"It is not based on good scientific data," said Dr. Bertha Madras, deputy director for demand reduction at the White House Office of National Drug Control Policy. "It's based on what some people would consider the right thing to do. But the studies supporting it are so sparse it's painful."

She pointed to a survey in 2003 of addicts in San Francisco. published in The Journal of Urban Health, in which 35 percent said they might feel comfortable using more heroin if they had naloxone on hand, and 62 percent said they might also feel less inclined to call 911.

"These were their attitudes," Dr. Madras said. "I'm taking the stand that in the absence of scientific evidence we don't engage in policies that would bring more harm than benefit."

Similar concerns were expressed by Dr. H. Westley Clark, director of the Center for Substance Abuse Treatment, a federal agency that finances treatment programs. "Our position is that naloxone should be administered by licensed health care professionals," Dr. Clark said.

Nevertheless, the direct-to-addicts model has spread rapidly since Chicago introduced it in the late 1990s. Baltimore, New York and San Francisco soon adopted the model, and Boston, Philadelphia, Connecticut, Minnesota, New Mexico, Rhode Island and Wisconsin have more recently joined the trend.

"The program here has been extremely successful," said Richard W. Matens, assistant commissioner of health for chronic disease prevention in Baltimore.

Overdose deaths there in 2005 were at their lowest level in more than a decade, and Mr. Matens gives at least some credit to the naloxone distribution.

The worrisome findings of the San Francisco survey have not been borne out by more recent studies of actual programs that include training in prevention and treatment.

A study in 2005 of San Francisco's pilot program found that of 20 overdoses witnessed by trained addicts, 19 victims received CPR or naloxone from the trainee, and all 20 survived. Knowledge about managing overdoses increased, and heroin use decreased.

"Research has shown none of the concerns about naloxone distribution to be true," said Dr. Sandro Galea, a researcher at the University of Michigan who has written two studies of programs in New York. "It probably is one of the few interventions that truly can reduce the deaths from opioids overdoses."

Dr. Herbert Kleber, who had Dr. Madras's position in the White House under President George H. W. Bush and now directs the Columbia University substance abuse division, said although he wished the evidence supporting naloxone distribution were stronger, "In terms of lives saved, it's probably the kind of intervention where there's a likelihood of more good than harm."

In New York City, the 863 overdose deaths in 2005 made up the fourth leading cause of death among people younger than 65, according to Dr. Thomas R. Frieden, commissioner of health and mental hygiene.

"We want people off drugs," he said. "But until they get off, we'd like them to stay alive. That means not getting H.I.V. and not dying of overdose."

Existing programs focus on reaching urban heroin addicts, but naloxone is equally effective at reversing overdoses from other opioids like OxyContin and methadone.

With overdose death rates from such drugs increasing sharply, officials in Wilkes County, N.C., are working on a program to dispense a naloxone nasal spray to users leaving hospital emergency rooms, detoxification centers and jails.

The program, Project Lazarus, received approval from the state medical board in November.

"Lazarus, biblically speaking, is one who was raised from the dead, and that is essentially what naloxone does for these people," said the director of the program, the Rev. Fred Brason II.

Dr. Sharon Stancliff, medical director of the Harm Reduction Coalition, which operates naloxone distribution and training in New York and San Francisco, conceded that the scientific case was not ironclad.

"Right now," Dr. Stancliff said, "we're at the point where we know it's safe. We're not seeing any bad outcomes.

"And we know it's feasible. We're just beginning to get really good evidence that it's associated with a significant reduction in overdose deaths."

Mark Kinzly, who is back in recovery after relapsing in 2005, says he has all the evidence he needs.

"This weekend I will go see my 9-year-old son play Pop Warner football," he said. "I am extremely grateful that the medication was available, and as a result I get to raise my child."

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