I think that There is a difference,
in the fact that you can get Mild Serotonin syndrome (which i believe i have from 200mg mdma then 9 days later doing 800mg)
Then there is the potientially fatal Serotonin Syndrome.
The symptoms are often described as a clinical triad of abnormalities:[1][5]
Cognitive effects: headache, agitation, hypomania, mental confusion, hallucinations, coma
Autonomic effects: shivering, sweating, hyperthermia, hypertension, tachycardia, nausea, diarrhea.
Somatic effects: myoclonus (muscle twitching), hyperreflexia (manifested by clonus), tremor.
Serotonin syndrome is a potentially life-threatening adverse drug reaction that may occur following therapeutic drug use, inadvertent interactions between drugs, overdose of particular drugs, or the recreational use of certain drugs
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http://en.wikipedia.org/wiki/Serotonin_syndrome