Illicit drug abuse continues to become more widespread, especially in teenagers. Therefore, it is important for physicians to recognize the signs and symptoms of abuse in their patients. Drug abuse should be considered in differential diagnosis of many physical and nearly all psychiatric complaints. An understanding of the pharmacologic mechanisms and adverse effects of illicit drugs can enhance overall care of patients who abuse drugs. The primary classes of drugs of abuse--cannabinoids, opiates, stimulants, hallucinogens, and inhalants--produce clinically diverse presentations. By recognizing these unique signs and symptoms, physicians can differentiate between drug-induced conditions any psychiatric illness.