Part of the potential danger is lousy control over the actual amount in the dose.
"Results show that melatonin content...with the actual content ranging from 83 percent less to 478 percent more than the concentration declared on the label."
> Further analysis with mass spectrometry also found serotonin, a much more strictly controlled substance, in 26 percent of the tested supplements. According to the authors, the presence of unlabeled but significant quantities of serotonin could lead to serious side effects.
wait, you can buy OTC seratonin? that seems wild given the seratonin syndrome risks.
edit: it looks like it's indirectly via 5-HTP [1].
"Results show that melatonin content...with the actual content ranging from 83 percent less to 478 percent more than the concentration declared on the label."
https://aasm.org/study-finds-that-melatonin-content-of-suppl...