Ernest Drucker, a pioneering public-health researcher who approached drug addiction with compassion, invigorated needle-exchange programs to stem the AIDS epidemic and diagnosed the destructive impact of what he called a “plague” of mass incarceration, died on Jan. 26 at his home in Manhattan. He was 84.

The cause was complications of dementia, his son, Jesse Drucker, said.

For more than three decades, Dr. Drucker, primed with epidemiological evidence, waged cutting edge campaigns to improve the lot of prison inmates; the homeless; patients with tuberculosis; workers exposed to asbestos; and HIV-infected drug users and their families, who had been ravaged by the repercussions of AIDS. He was an early and vocal proponent of rethinking the country’s approach to illicit drugs, advocating “harm reduction” — a strategy that prioritizes reducing negative consequences over criminal prosecution.

A clinical psychologist by training, he was professor emeritus of family and social medicine at Montefiore Medical Center/Albert Einstein College of Medicine in the Bronx and had been a senior research associate and scholar in residence at John Jay College of Criminal Justice of the City University of New York in Manhattan, where he biked to work from the Upper West Side.

Dr. Helene Gayle, an epidemiologist and a former president of Spelman College in Atlanta, described Dr. Drucker this way in an email to his son: “Unapologetic about taking on issues that others wouldn’t touch. Unapologetic about the humanity in all including those who had suffered the most injustice.”

Having run a drug rehabilitation program in the Bronx, Dr. Drucker knew firsthand the destructive capabilities of addictive drugs. But the criminal prosecution of addicts, he argued, only compounded the problem, forcing addicts underground, where dangerous practices like sharing needles resulted in the spread of H.I.V., and saddling them with criminal records that could make them unemployable.

“Our demonization of heroin has transformed otherwise benign and controllable patterns of its use into a lethal gamble and has raised the threshold for seeking help when problems do arise,” he wrote in a letter to The New York Times in 1995. “Other countries are adopting ‘harm reduction’ strategies that (without legalizing drugs) acknowledge their widespread use and employ methods (such as needle exchange) to make even injectable use safer.”

He added that “our stubborn failure to acknowledge the enduring appeal of drugs, and learn how to control (rather than prohibit) their use” had tragic consequences.

Instead, he contended in his book “A Plague of Prisons: The Epidemiology of Mass Incarceration in America” (2011), that for the 90 percent of drug crimes that are nonviolent “criminalization can be replaced with a public health and therapeutic model.”

Dr. Drucker established some of the world’s first syringe exchange programs and in 1991, after he testified in favor of them, four members of the AIDS Coalition to Unleash Power, known as Act Up, were acquitted in Jersey City, N.J., of charges related to running a needle exchange program to prevent the spread of AIDS.

He warned that the AIDS epidemic was not only afflicting gay men but was also increasingly devastating poor, nonwhite families. He said that heterosexual relations in certain Bronx neighborhoods had become a form of “sexual Russian roulette” that was orphaning children.

“No one, by a long shot, spun out more novel ideas for research, policy and advocacy, many of which resulted in influential publications, new organizations and changes in policy on all aspects of harm reduction as well as drug treatment, public health and criminal justice reform,” Ethan A. Nadelmann, the founder of the Drug Policy Alliance, which has opposed the war on drugs, said in an email.

Dr. Drucker was a founder and chairman of Doctors of the World/USA from 1993 to 1997, was the founding editor in chief of Harm Reduction Journal, and a founder of the International Harm Reduction Association.

In his eulogy, Professor David Michaels of George Washington University’s Milken Institute School of Public Health and a former administrator of the federal Occupational Safety and Health Administration, said Dr. Drucker was “a true public health Renaissance man, driven by a deep commitment to fairness, and justice. He cared deeply about people — you can see it in his work on harm reduction, insisting we should not punish people for who they are and the decisions they make, but instead we should help them become healthier and more fulfilled.”

Ernest Mor Drucker was born on March 29, 1940, in Brooklyn. His father, Joseph, was a machinist for ITT. His wife, Beatrice (Strull) Drucker, managed the household.

Ernest was raised in the borough’s Brighton Beach section, and graduated from Brooklyn Technical High School with plans to become an engineer, but he gravitated toward psychology as a major at City College of New York, where he earned a bachelor’s degree in 1962 followed by a doctorate.

At Montefiore/Einstein, where he was a disciple of Drs. H. Jack Geiger and Victor Sidel, two proponents of health care for the poor, he was the director of Public Health and Policy Research and the founding director of a 1,000-patient drug treatment program, where he served until 1990.

He would periodically reinvent himself professionally, shifting his focus to and from heroin addiction, public health, occupational safety, AIDS and alternatives to prison.

In addition to his son, Jesse, an investigative reporter for The New York Times, he is survived by his wife, Jeri (Rosner) Drucker, an artist; his brother, Alan Drucker; and two grandchildren.