Oral formulations prepared from the leaves of the kratom (Mitragyna speciosa) plant are increasingly used for their opioid-like effects to self-manage opioid withdrawal and pain. Calls to US poison centers involving kratom exposures increased >50-fold from 2011–2017, one-third of which reported concomitant use of kratom with drugs of abuse. Many of these drugs are eliminated primarilyviacytochrome P450 (CYP) 3A and CYP2D6, raising concerns for potential adverse pharmacokinetic kratom-drug interactions. The impact of a single low dose of kratom tea (2 g) on the pharmacokinetics of the CYP3A probe midazolam (2.5 mg) and CYP2D6 probe dextromethorphan (30 mg) were assessed in 12 healthy adult participants after oral administration. Kratom showed no effect on dextromethorphan area under the plasma concentration time-curve (AUC) and maximum concentration (Cmax; geometric mean ratio (90% confidence interval) 0.99 (0.83–1.19) and 0.96 (0.78–1.19), respectively) but a modest increase in midazolam AUC andCmax(1.39 (1.23–1.57) and 1.50 (1.32–1.70), respectively). Lack of change in midazolam half-life (1.07 (0.98–1.17)) suggested that kratom primarily inhibited intestinal CYP3A. This inference was further supported by a physiologically based pharmacokinetic drug interaction model using the abundant alkaloid mitragynine, a relatively potent CYP3A time-dependent inhibitorin vitro(KI, ~4 μM;kinact, ~0.07 min−1). This work is the first to clinically evaluate the pharmacokinetic drug interaction potential of kratom. Co-consuming kratom with certain drugs extensively metabolized by CYP3A may precipitate serious interactions. These data fill critical knowledge gaps about the safe use of this increasingly popular natural product, thereby addressing ongoing public health concerns.