WASHINGTON  His son had been dead from an overdose only three months when A. Thomas McLellan, among the nation’s leading researchers on addiction, got a call from the office of Vice President Joseph R. Biden Jr. Would he accept the nomination to be the government’s No. 2 drug-control official?

Dr. McLellan, 61, makes no secret of his cynicism about government  “I hate Washington,” as he put it in an interview  and he had no intention of leaving his job as a professor of psychology at the University of Pennsylvania School of Medicine and scientific director of the Treatment Research Institute in Philadelphia.

But the loss of his younger son, who overdosed on anti-anxiety medication and Scotch last year at age 30 while his older son was in residential treatment for alcoholism and cocaine addiction, changed his perspective.

“That’s why I took this job,” said Dr. McLellan, who was sworn in as the deputy director of the Office of National Drug Control Policy in August. “I thought it was some kind of sign, you know. I would never have done it. I loved all the people I’ve worked with, I loved my life. But I thought maybe there’s a way where what I know plus what I feel could make a difference.”