His appointment as deputy to the nation's so-called drug czar energized a field that has been repeatedly disappointed by administrations of both parties that emphasized law enforcement at the expense of treatment and prevention.
"We will have a great hole," said Sue Rusche, president and CEO of National Families in Action, an organization that seeks to prevent drug abuse among children by promoting science-based policies. "He is one of our most enlightened people who understands the complexity" of drug addiction.
McLellan, a leading scientist in the field of drug addiction, cofounded the Treatment Research Center in Philadelphia in 1992 with the goal of moving proven treatment practices from the laboratory to the clinic. He has been a major force in drug treatment for decades.
People close to him said that when he was recruited by Vice President Biden in early 2009, he committed to one year.
"The last time I talked with the vice president they were prevailing upon him to stay," said Charles P. O'Brien, a mentor and director of the Center for Studies of Addiction at the University of Pennsylvania School of Medicine. "I am going to offer him a job this afternoon."
He is not likely to accept it, at least not immediately, said his wife, Deni Carise, who took a job last fall as chief clinical officer for New York-based Phoenix House, a national operator of treatment centers. She and her husband have been meeting on weekends in Philadelphia at their home in the Bella Vista section.
"He wants to boat around for a few months," she said.
The death of his 30-year-old son from a combination of antianxiety medication and alcohol poisoning two years ago was a factor in McLellan's decision to accept the D.C. job.
"He thought it would help him make sense of that loss," Carise said, but the separation from friends and family "made that loss worse."
McLellan, 62, was attending an American Society of Addiction Medicine conference in San Francisco and did not return calls requesting comment Friday. His decision, which had been rumored for weeks, was first reported by the newsletter Alcoholism & Drug Abuse Weekly.
"There's no deep dark secret here - I'm just ill-suited to government work," McLellan said in an interview Thursday with editor Alison Knopf.
Although McLellan is credited with raising the level of sophistication on treatment issues in the drug czar's office, some have accused the Obama administration of saying all the right things but not backing them up with action or money. Release of the National Drug Control Strategy, the blueprint for how all federal agencies deal with drug issues, has been repeatedly delayed, leaving few details by which to judge.
Politics is constantly threatening to intrude on decisions about addiction policy that should be based on science, said Ethan Nadelmann, founder and executive director of the Drug Policy Alliance, which promotes alternatives to the "war on drugs."
"McLellan's departure is not reassuring," said Nadelmann, who had bluntly questioned the administration's commitment at a congressional hearing Wednesday.
Gil Kerlikowske, the drug czar and McLellan's boss, who also testified, has publicly dismissed the term war on drugs. In an interview Friday, he said that treatment would be emphasized and that he believed strongly in the policies advocated by McLellan.
"I want to do everything I can to hold on to him as long as I possibly can," he said.
Contact staff writer Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.