Amazon recently announced that it will no longer include marijuana in the company’s drug screening program for new hires (other than for positions regulated by the Department of Transportation). Amazon says marijuana will instead be treated like alcohol, which means “impairment checks” on the job site and testing for all drugs and alcohol after job incidents.

Drug screening and testing are widespread in the US, covering nearly half of all workplaces. But Amazon’s announcement, and the recent spate of marijuana medicalizations and legalizations, have prompted discussion of who should decide whether such programs make sense.

The federal government started the push for employee drug testing when it collected urine samples from Vietnam War veterans (to screen for abusers and assist rehabilitation). Following a series of incidents, the U.S. Navy announced a policy of “zero tolerance” for illicit drugs in 1980. By 1985, major corporations had adopted drug screening and testing programs. The next year, the Reagan administration instituted urine testing in federal agencies. In 1988, the Drug-Free Workplace Act required federal contractors to establish and maintain drug-free policies. Safety and security-sensitive occupations are also subject to specific drug-testing legislation.

Evidence suggests drug testing indeed deters drug use in the workplace. Interestingly, a 2014 paper also finds that drug screening benefits African Americans because, absent screening, employers tend to assume minority applicants use drugs. But good evidence on whether drug tests lead to higher productivity or safer workplaces is scant.

Companies’ revealed preferences indeed suggest screening and testing programs are useful. But why? As mentioned before, the evidence that testing generates higher productivity or enhanced safety is tenuous. Another possibility is that drug use correlates with lack of self-control, poor judgment, and other undesirable attributes. Indeed, the popularity of preemployment screening, as opposed to on site testing, suggests drug programs may provide useful information on prospective employees.

Whatever the merits of testing programs, companies are in the best position to decide whether the programs are worth the costs. Each job requires different tasks, attributes, and levels of readiness; the benefits of preventing drug abuse are presumably higher for an airline pilot than for most office jobs. Costs may also differ based on frequency and type of test, plus some kinds of workers may have a particular aversion to drug testing.

Many civil libertarians express concerns about drug testing, and they are right to oppose government imposition of such testing. Relatedly, private drug testing may have expanded excessively due to not-so-subtle government pressure. But so long as government keeps its thumb off the scale, private employers should be free to test, or not, as they see fit.