Rationale Kratom ( Mitragyna speciosa ), a plant native to Southeast Asia, has become popular in the United States for its reported ability to treat substance use disorders (SUD). Mitragynine (MG) is the most abundant alkaloid found in kratom. Objectives The present studies evaluated whether MG decreases methamphetamine self-administration under both fixed-ratio (FR) and progressive-ratio (PR) schedules. In a separate cohort w...