5-HTP increases a brain chemical called serotonin. Taking 5-HTP along with other herbs and supplements that increase serotonin might lead to too much serotonin and cause side effects including heart problems, shivering and anxiety. Other herbs and supplements that increase serotonin levels include Hawaiian baby woodrose, L-tryptophan, S-adenosylmethionine (SAMe), and St. John's wort.

An interesting concept that has emerged from initial findings is that regeneration and fibrosis are competing events in the vertebrate heart. That is, if there is a capacity for injury-stimulated cardiomyocyte hyperplasia beyond a certain threshold, regenerative mechanisms will overcome scarring. Results consistent with this idea came from experiments with zebrafish possessing a ts mutation in the cell-cycle checkpoint kinase Mps1 (Poss et al., 2002b). As mentioned earlier, mps1 mutants were initially identified based on their defects in caudal fin regeneration. Serendipitously, mps1 mutants also showed defects in cardiac regeneration at a temperature restrictive for the mutation (Poss et al., 2002b). Instead of regenerating muscle in response to ventricular resection injury, mps1 mutants repaired wounds by forming large, collagen-rich scars. Inhibition of Fgf signaling also stunts cardiac regeneration and causes scarring (Lepilina et al., 2006). These results indicate that even vertebrates with high cardiac regenerative capacity have a default scarring mechanism; normally, regeneration somehow restricts this pathway (Fig. 8). The implication is exciting; perhaps by stimulating regeneration in a poorly-regenerative system like the mammalian heart, scarring events characteristic of myocardial infarction would be restricted by new muscle formation. Similarly, deterring cardiac scarring mechanisms would perhaps favor regeneration in mammals.
Johansson, A., Westberg, L., Sandnabba, K., Jern, P., Salo, B., & Santtila, P. (2012). Associations between oxytocin receptor gene (OXTR) polymorphisms and self-reported aggressive behavior and anger: Interactions with alcohol consumption [Abstract]. Psychoneuroendocrinology 37(9), 1546-56. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/22421562
A 2002 review concluded that although the data evaluated suggests that 5-HTP is more effective than placebo in the treatment of depression, the evidence was insufficient to be conclusive due to a lack of clinical data meeting the rigorous standards of today.[2] More and larger studies using current methodologies are needed to determine if 5-HTP is truly effective in treating depression.[3][4] In small controlled trials 5-HTP has also been reported to augment the antidepressant efficacy of the antidepressant clomipramine.[5][6][7]