I never tried it but my daughter did, and it made her really sick. She's always suffered from PTSD nightmares and I seem to recall they were intensified too, like Misslizard said.
I think it only works if your body is actually low on Melatonin, and if you can metabolise the tablets.
If it affected your stomach it's probably because Serotonin is produced in the stomach. I know Melatonin isn't the same as Seratonin, but Melatonin is absorbed in the GT tract. Maybe you had a double-whammy.
I hope you feel better, Sarah.
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And in the end, the love you take is equal to the love you make.