One problem associated with controlled substance screening involves separating illegal drug use from legal prescription drug use and identifying whether an employee who tests positive for a drug is taking the drug legally or illegally. Additionally, managers should know that it is perfectly legal to be under the influence of controlled substances, even if they are illegally obtained. Still other problems involve the reliability of drug tests, the competence of laboratories conducting the drug tests, the integrity of the chain of custody of the evidence, and the relationship between positive test results and job impairment. Falsely accusing employees as the result of testing error can result in libel suits. Employers who institute mandatory drug testing programs should realize that random blanket screening of employees lowers morale. Employers should establish drug testing programs for employees only if an employee performance problem exists or if undetected drug use by employees would endanger coworkers or threaten public safety. The article recommends general policies for organizations to use in determining whether and how to test employees for substance abuse. 42 footnotes.