More than four out of five U.S. employers now require pre-employment drug tests, and 39 percent conduct random drug testing of employees, the Lakeland (Fla.) Ledger reported Feb. 6.

The Society for Human Resource Management said in a 2006 report that 84 percent of private employers conduct pre-employment testing, 39 percent conduct random screening of employees, 73 percent conduct for-cause testing, and 58 percent require drug tests after on-the-job accidents. State and federal law also requires drug testing in many public-sector jobs.

The tests cost about $40 each. Some employers see it as money well-spent, but critics say the tests are intrusive and ineffective. Experts note, for example, that the tests are far more likely to detect marijuana, which stays in the body for up to a month, than harder drugs like cocaine and heroin, which are metabolized within one to three days. And few employers test for alcohol.

"A policy that screens out marijuana users while allowing drinkers is arguably counterproductive, since alcohol intoxication -- unlike marijuana -- produces hangovers that can significantly impair job performance the next day, and alcohol is by any measure far more toxic and far more addictive than marijuana," said Bruce Mirken, a spokesman for the Marijuana Policy Project.

But Florida manufacturing company Keymark says its testing regimen -- which gives some employees whop test positive a chance to attend a treatment program -- helps promote workplace safety. "We don't want anybody high on drugs around here operating our equipment," said company spokesperson Ron Waite. "Our program I think serves its purpose. It serves as a deterrent. Obviously we're not going to catch everybody, but we think it's a worthwhile program."