As reported in February 2018, the FDA has confirmed from analysis that kratom has opioid properties. More than 20 alkaloids in kratom have been identified in the laboratory, including those responsible for the majority of the pain-relieving action, the indole alkaloid mitragynine, structurally related to yohimbine. Mitragynine is classified as a kappa-opioid receptor agonist and is roughly 13 times more potent than morphine. Mitragynine is thought to be responsible for the opioid-like effects.

Kratom, due to its opioid-like action, has been used for treatment of pain and opioid withdrawal. Animal studies suggest that the primary mitragynine pharmacologic action occurs at the mu and delta-opioid receptors, as well as serotonergic and noradrenergic pathways in the spinal cord. Stimulation at post-synaptic alpha-2 adrenergic receptors, and receptor blocking at 5-hydroxytryptamine 2A may also occur. The 7-hydroxymitragynine may have a higher affinity for the opioid receptors. Partial agonist activity may be involved.

Additional animal studies have suggested that these opioid-receptor effects may be reversible with the opioid antagonist naloxone.

Time to peak concentration in animal studies is reported to be 1.26 hours, and elimination half-life is 3.85 hours. Effects are dose-dependent and occur rapidly, reportedly beginning within 10 minutes after consumption and lasting from one to five hours.