^i dont think so. Now im little hesitent to talk on this because i deffinitely dont understand the brain chemistry of psychs myself... but if I understand the way psychedelics bind to receptor sites correctly, increased serotonin levels (from the 5-htp) would actually compete with the psychedelic, hence making it weaker... I think, dont quote me on that, but I will say 5-htp deffinitely does not make a trip stronger or more visual
I find this topic real interesting though, its kind of like "anti-hppd". I think the most important thing that needs to be learned about HPPD is whether it is the result of permenent or temporary changes in brain function, and if so at what point are the lines crossed, or rather is it from damage to the photoreceptors in the eye, and if so does it eventually repair itself or return to normal. Theres the classic line regarding neurochemistry "cells that fire together, wire together", however over time I guess these connections can be weakened and slowely returned to normal, or change to something else at least...
Also, Im pretty sure ive seen a memeber on here named "Xorkoth" say he does not get visuals any more (big apologies if im getting users mixed up)... And I think Ive also seen him hint at his past psychedelic use (which I think most would consider pretty heavy) as the culprit.
Over my tripping career Ive deffinitely noticed a change in my visuals, how they act, what they do. Whether this is the result of changes in brain functioning, perment degrading of the eyes photoreceptors, getting "use to" the visual effect, tolerence, or perhaps that the same chemical can play on the senses in different ways at different times, changing even more depending on set and setting, well who can say. To be quite honest Ive got a case of HPPD myself, mainly as afterimages, these "black lines" that are very reminiscent of floaters, even behave as you expect floaters but clearly are not, and occassional what I call "color washing" over my eyes if a stare at a point for a few seconds then move my eye just a bit. sometimes its better, some times it worse, but it never really goes away.