Malaysia and southern Thailand. In the U.S., , kratom was available in outlets such as head shops and over the Internet; the prevalence of its U.S. use was unknown at the time. In the United States, kratom use increased rapidly between 2011 and 2017. By 2020, it was estimated that 15 million people in the U.S. use kratom. Adverse effects Mitragyna speciosa may cause many adverse effects and in November 2017 the FDA issued a public health advisory for the drug. The side effects of kratom appear to be dose-dependent and are more common with doses that exceed 8 g. While the incidence of adverse effects in people who use kratom is unknown, a 2019 review of 935 kratom exposures reported to U.S. poison control centers over a seven-year period listed the following signs and symptoms: agitation (18.6%), tachycardia (16.9%), drowsiness (13.6%), vomiting (11.2%), confusion (8.1%), seizure (6.1%), withdrawal (6.1%), hallucinations (4.8%), respiratory depression (2.8%), coma (2.3%), and cardiac or respiratory arrest (0.6%). The study also reported two deaths and four cases of neonatal abstinence syndrome. A different 2019 review listed as common side effects: decreased appetite, anorexia, weight loss, temporary erectile dysfunction, insomnia, sweating, hyperpigmentation, hair loss, tremor, and constipation. Kratom products in the U.S. are commonly used in doses ranging from 2–6 g of dried leaf per dose, and doses exceeding 8 g are relatively uncommon. Given that kratom products may vary greatly in potency, there is no standard dosing system. At relatively low doses (1–5 g of raw leaves), at which there are mostly stimulant effects, side effects include contracted pupils and blushing; adverse effects related to stimulation include anxiety and agitation, and opioid-related effects like itching, nausea, loss of appetite, and increased urination begin to appear. At moderate to high doses (5–15 g of raw leaves), at which opioid effects generally appear, additional adverse effects include tachycardia (an increased stimulant effect) as well as the opioid side effects of constipation, dizziness, hypotension, dry mouth, and sweating. Long-term use of high doses of kratom may lead to development of tolerance, dependence, and withdrawal symptoms, including loss of appetite, weight loss, decreased sexual drive, trouble sleeping, muscle spasms, muscle and bone pain, myoclonus, watery eyes, hot flushes, fever, diarrhea, restlessness, anger, and sadness. This may lead to resumption of use. Frequent use of high doses of kratom may cause tremors, anorexia, weight loss, seizures, and psychosis. However, in