BOSTON - Rush Limbaugh is not alone. Addiction to prescription painkillers has boomed in recent years, and they can be as tough to kick as heroin. The number of Americans who begin misusing painkillers each year has almost quadrupled from 1990 to 2001, according to government figures. And many abusers don't recognize the insidious slide into addiction. "It's just so much more acceptable in society for people to be taking prescription drugs," said Sean Evans, 31, of Everett, Mass., a construction worker who became addicted to the pain reliever OxyContin, then moved on to heroin. "You can always rationalize the...