Random drug testing in the workplace has become more common since federal guidelines were issued in 1988, despite the criticism that most positive tests are the result of occasional use of illicit drugs. In order to determine the relative probabilities of detecting frequent versus infrequent users of illicit drugs, a survey of 15 experts in the drug abuse field was conducted. Based on the responses, it was estimated that 55% of employed people who used any illicit drugs in the prior year were annual drug users, 37% were monthly users, and 8% were daily users. Analysis using probability theory indicated that among workplace drug users who test positive, 52% will be daily users, 41% will be monthly users, and only 7% will be annual users. At a 50% testing rate, random drug tests identify 40% of daily users, 8% of monthly users and only 1% of annual users during the course of a year. The estimated rate of illicit drug use among employees is approximately eight times the average random testing positive rate. Random drug tests in the workplace are effective in identifying near daily users of illicit drugs, but they are less effective at identifying infrequent drug users. Employers have found that random drug testing is a deterrent to both frequent and occasional use of illicit drugs.