Marijuana (cannabis) is the most frequently used illicit drug of abuse in the United States and worldwide. Moreover, it is second only to alcohol as the most prevalent psychoactive substance seen in cases of driving under the influence of drugs. It is also by a wide margin, the drug most often detected in workplace drug-testing programs. The primary psychoactive substance in marijuana is delta-9-tetrahydrocannabinol, known simply as THC. This guidance summarizes current evidence regarding marijuana consumption, discusses possible side effects including temporary impairment as it relates to the workplace, reviews existing federal and state laws and legal implications for health care professionals and employers, and suggests various strategies available to employers for monitoring workers for marijuana use. It is outside the scope of this article to address any potential medical benefit of marijuana.