Both mitragynine and 7-hydroxy bind to mu-opioid receptors. These are the same receptors activated by morphine, oxycodone, and heroin. But while mitragynine is considered a partial agonist, 7-hydroxymitragynine is significantly stronger in its binding affinity.
Animal studies have shown that 7-hydroxymitragynine may be up to 13 times more potent than morphine. Its effects kick in fast, bind hard, and leave a longer tail of dependency than mitragynine alone. This is why some kratom users feel like they are not just dealing with caffeine jitters or a mild comedown. They describe full-blown withdrawal symptoms, even if their use started as occasional or casual.
The addictive profile of 7-hydroxy explains why kratom extracts are especially difficult to quit. These extracts often contain artificially boosted levels of 7-hydroxymitragynine. Some shots and capsules contain more 7-hydroxy than the entire natural leaf profile ever could. The result is a faster buildup of tolerance, more intense cravings, and a more painful crash.