The study aim is to evaluate potential analgesic properties of ketum using the cold pressor task.

Participants in the proposed study will be individuals who report consuming ketum more than once a day, daily for the past 12 months. The study is a placebo-controlled, double-blind within-subject repeated mesaures design. Participants will ingest a sequence of ketum and placebo drinks prepared by pharmacologists at USM Centre for Drug Research and their pain tolerance will be measured by the cold pressor task after ingesting ketum or placebo.

Inclusion Criteria: Individuals who report drinking ketum decoction more than once a day, daily for the past 12 months.

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Exclusion Criteria: (1) tested positive for illicit drug use including morphine, cannabis, methamphetamine, amphetamine, methadone, benzodiazepine and ketamine; (2) have current or previous history of psychological or neurological problems (self-report); (3) have current or previous alcohol use history (self-report); (4) HIV infections; (5) history of chronic liver disease (e.g. cirrhosis, hepatitis A, B, C, etc.) 6) non-alcoholic fatty liver disease; (7) history of coronary heart disease and diabetes; (8) existing chronic pain condition and (9) history of psychiatric disorders. Exclusion criteria for female particiapns: (1) pregnancy, (2) nursing.

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Ketum and placebo drinks will be matched for taste and appearence. Each study participant will receive a sequence of ketum and placebo drinks in random order.