Nobody knows exactly what causes serotonin syndrome however the majority of people who have serotonin syndrome have in their past used anti depressants (SSRIs, MAOIs) concurrently with another medication that would trigger the release of serotonin. The problem in pinpointing down these exact chemicals is that it doesn't take something as strong as MDMA to release enough serotonin to cause the syndrome, but something as miniscule as Benadryl, Ephedrine, DXM...things we use everyday without thinking twice about it. Now a lot of OTC medication bottles will have written that it is contraindicated with a MAOI (I believe that's what it says), but in reality the list is much longer than MAOIs.
Take for example, recently I've decided that I would like to try and give 5-HTP a go for 1-2 months and see what results I would get. I never ended up taking it due to what it said on the bottle.
"Consult a health care practitioner prior to use if symptoms persist or worsen or if you are taking carbidopa or drugs/supplements with serotonergic activity such as L-tryptophan, 5-adenosylmethionine, St. Johns wort, antidepressants, pain killers, OTC cough and cold medication containing dextromethorphan, anti-nausea and anti-migraine medications. Discontinue use and consult your healthcare practitioner if you show signs of.......etc, etc."
I met a person with serotonin syndrome once, while attending a depression group. Starting with someone in the circle we had formed we began a process by which if we wanted to speak we would introduce ourselves to the group, speak & answer questions about depression related issues in our lives, some spoke for seconds, others for minutes, and yet others that weren't ready would simply say "pass". It came the turn of a girl who was a good looking girl too, I kept looking at her and she looked like she'd laugh/start to laugh, she said pass, after about half hour we had gone full circle and it was decided to go again, this time more comfortable with each other. The girl's turn came this time and she began to speak. Not 1min in her story as she starts to speak she cannot make it to utter more than 3-4 words at best without starting to cry uncontrollably. Only then do I realize that what looked like she was smiling/laughing (or about to) was her holding back her tears. She said she had serotonin syndrome and that doctors didn't know what caused it, that she's seen several doctors and since no one knows the exact cause there is no current treatment. She cried her words...literally.
