Workplace drug testing has become an increasingly common practice. Today, 98 percent of America’s largest companies use drug testing to screen potential employees, and many firms conduct random drug tests of their workers.

Proponents of employee drug testing argue that such tests reduce and deter substance abuse among workers, improve workplace safety and productivity, and reduce health-care costs attributed to drug abuse. According to Drug Watch International, “Drug testing is an effective and humane method to deter and to detect drug use. It has been overwhelmingly supported by the courts.” Drug Watch International and other advocates credit drug testing for helping reduce workplace drug abuse by half between the mid-1980s and the mid-1990s.

But critics maintain that workplace drug testing violates employees’ privacy, is often inaccurate, and does not reveal whether employees are actually impaired while...