Few issues are as vexing to corporate managers these days as the controversy over drug testing of their employees -- and potential employees. The great majority of bosses find themselves somewhere in the middle of the opinion range, between those who are sure that insisting on evidence that workers are completely drug free is simply good management and those who feel that it is an impermissible intrusion in private lives to even raise the question without reasonable suspicion of a problem.

The issue is going to be increasingly tough to evade, according to a report in the latest issue of HRMagazine, the journal of the Society for Human Resource Management. The article cites three factors that will goad employers toward drug testing of employees in the decade ahead:

More use of drugs "within all segments of our society."

Unreliability of reports from former employers, who may mask a worker's drug use out of fears of litigation.

Vulnerability of the new employer to lawsuits for "negligent hiring" if a drug-using employee causes harm.

There are moral and social components of the testing dilemma, but it would be easier to make a decision if the legal standards were clearer.

At the moment, "random testing in the private sector may possibly conflict with individual privacy rights guaranteed by state constitutional provisions, by state and local laws, and by provisions in collective bargaining agreements," the law firm Jackson, Lewis, Schnitzler & Krupman told clients in a recent bulletin.

That makes it tough for companies operating in a number of jurisdictions to work out a policy that can be applied at all locations.

For instance, this spring a mid-level New York State appeals court ruled that to use a drug test, employers must have solid evidence that the test is "a valid predictor of job performance." State laws bar discrimination against the disabled, including those perceived to be disabled, unless they can't do the job.

That covers drug users, the opinion in Doe v. Roe found. The judges said a company is within its rights to assume that drug users are unfit, but it must be able to show that the tests used really pinpoint drug users.

That opened the way for a trial on the claims by a financial analyst that the immunoassay that showed opiates in his urine actually had picked up the components of a health bread he ate in great quantities.

The New York State case was particularly startling to business because the financial analyst had never gotten the job: The conditional offer was withdrawn after the test results came back from the lab.

Prevailing legal wisdom is that curbs on drug testing apply to employees only, not to job applicants. That theory is based not on discrimination law but on labor law.

Last year, the National Labor Relations Board turned down a union request that the Minneapolis Star Tribune negotiate over its policy of insisting on a pre-employment drug test -- and rejecting those who flunk. The NLRB had just ruled that it is an unfair labor practice for a company to unilaterally begin a drug testing program: It's a question that must be bargained over with the union.

But in the newspaper case, the board said a union's right to insist that an employer talk over personnel policies applies only to actions that affect current employees -- not their potential co-workers.

For those already on the payroll, "there should be a reasonable basis for probable cause before an employee is tested," the HRMagazine article advises. Otherwise, a company is courting legal troubles, unless the jobs have such an impact on public safety that unannounced random testing of workers seems prudent.

The "probable cause" standard focuses on the behavior of the individual employee, and is invoked only when absenteeism or poor performance suggests that the worker has a hidden problem.

If existing legal decisions do not provide business any clear guidance on whether to do drug testing, neither does the government.

Two years ago, Congress passed the Drug-Free Workplace Act, intended to make sure that federal spending went only to employers who keep drugs out of their offices and factories. But in March when the Office of Management and Budget issued a series of questions and answers intended to help contractors comply with the law, the agency pretty much left it up to each CEO.

Drug testing of employees might be part of a company's anti-drug program, the OMB noted, along with health education and help for workers who do get hooked. But the guide emphasizes that neither testing nor any other component is a required ingredient. All that's mandatory is that some program addressing the issue be in place.

Daniel B. Moskowitz is a Washington editor for Business Week newsletters.