I believe so - I was never officially diagnosed, but became extremely ill and became better pretty much instantly on withdrawal of the 5-HTP. The symptoms matched those of serotonin syndrome, so I believe I had it - if I did, I was very lucky, as it can be fatal. I was taking, at the time, two over-the-counter 5-HTP supplements per day (can't remember the dosage, but they were fairly standard ones), but I was combining it with a lot of coffee - maybe ten, fifteen cups a day? - and that can have a synergetic effect when combined with serotonin precursors, as caffeine acts on the serotonergetic system.