Drug testing in the workplace peaked in 1996, when 81 percent of employers screened workers and pre-employment prospects for illegal substances, reports the American Management Association.

The practice has declined gradually in recent years, dropping to 62 percent of employers in 2004 amid privacy concerns and a job market in which some companies don't have the luxury of being too picky.

In 1987, 22 percent of companies tested for drugs, according to the management association. Then concerns over safety, community image and a desire to decrease employee absenteeism and increase worker productivity prompted a swell of employers to start testing.

Nearly 8 percent of full-time workers ages 18 to 49 use illegal drugs, according to the Department of Health and Human Services' Substance Abuse & Mental Health Services Association.

Industry studies show each drug user in the workplace can cost an employer $11,000 to $13,000 annually because of healthcare costs, lost production, injuries and damage to equipment. But drug tests can be expensive, too, ranging from $4,400 for urinalyses of 100 employees to $10,000 for hair-sample testing.