Kratom (Mitragyna speciosa) is a plant indigenous to Thailand and Southeast Asia. Kratom leaves produce complex stimulant and opioid-like analgesic effects. In Asia, kratom has been used to stave off fatigue and to manage pain, diarrhea, cough, and opioid withdrawal. Recently, kratom has become widely available in the United States and Europe by means of smoke shops and the Internet. Analyses of the medical literature and select Internet sites indicate that individuals in the United States are increasingly using kratom for the self-management of pain and opioid withdrawal. Kratom contains pharmacologically active constituents, most notably mitragynine and 7-hydroxymitragynine. Kratom is illegal in many countries. Although it is still legal in the United States, the US Drug Enforcement Administration has placed kratom on its "Drugs and Chemicals of Concern" list. Physicians should be aware of the availability, user habits, and health effects of kratom. Further research on the therapeutic uses, toxic effects, and abuse potential of kratom and its constituent compounds are needed.