It could have been the media stories from about 30 years ago—a tryptophan supplement made a lot of people very ill, and some of them died. It was thought to be due to a contaminant, and I don't think anything like it has happened since.
I read those stories years ago and the stories I heard were...
In the late 80s or early 90s a lot of tryptophan supplements were made in Japan. One company tried to create a new method of developing tryptophan that would save them money but something went wrong - some substance in the process got contaminated with bacteria. As a result there were a lot of deaths around the world, particularly in the USA where tryptophan was very popular as a cheap non-prescription anti-depressant. Inspectors tried to find out what had gone wrong but the company cleaned up and quickly removed all evidence of their new process for developing tryptophan. With no evidence left nothing much could be learned.
https://en.wikipedia.org/wiki/Tryptophan#Showa_Denko_contamination_scandal
For quite a long time tryptophan was banned around the world, and it was because of that that 5-HTP began to be used instead. 5-HTP is one of the products that tryptophan is converted into in the body.
See this diagram :
https://upload.wikimedia.org/wikipedia/commons/2/24/Tryptophan_metabolism.svg
For probably murky reasons to do with Big Pharma profits from SSRIs, 5-HTP got tarred with the same brush as tryptophan even though it hadn't ever been contaminated and it helped a lot of people. I remember when I first heard about 5-HTP many years ago there was a peculiar man on the internet who was doing his best to ruin the reputation of 5-HTP and get it banned for some unknown reasons of his own. He ranted about 5-HTP a lot.
Tryptophan was "unbanned" some time ago in the UK but I don't know exactly when.
I know that some 5-HTP products are combined with vitamin B6 for "reasons" but I don't know what they are. I've never bought a combined B6 and 5-HTP product myself.
Regarding serotonin syndrome, yes, 5-HTP can cause it, as can many other prescribed medications, supplements, and drugs when taken at high doses or in combination.
https://en.wikipedia.org/wiki/Serotonin_syndrome#Causes
Some companies selling 5-HTP now are creating the conditions that could get 5-HTP banned in future. They are creating products with higher and higher doses. When I first bought 5-HTP most products were sold in a dose of 50mg - 200mcg per tablet/capsule. Since then the doses have got higher and higher and it is actually quite hard to find products containing only 50 mcg, while some dangerous people are selling 800mg doses. I suspect that the number of cases of serotonin syndrome are going to rise and Big Pharma will want it banned.
I've been taking 5-HTP since 2011 at a low dose of 50mg per night. Higher doses give me the jitters. 5-HTP is known to raise cortisol, and since mine is already high it is probably why I can't take more than 50mcg. I decided about 2 or 3 years ago that I wanted to cut down on my dosage of 5-HTP, and I have done that without any evidence of a withdrawal reaction, so it isn't addictive, in my opinion.
