I recently tried 5HTP and later L-Tryptophan and caused the worse IBS attack I have had in years. I posted here and someone told me about the connection between serotonin and IBS. I also found a good description of the mechanism for this here.
I'm kicking myself because I haven't had it that bad for a long time. I felt like maybe my gut was healing and now I have to start back at square one.