An open label clinical study published in 2017 assessed the benefit of a combination of creatine and 100 mg of 5-HTP twice daily, in combination with already approved SSRIs or SNRIs (selective norepinephrine reuptake inhibitors). Supplementation was given for 8 weeks, with 4 weeks of post treatment follow up, with the primary endpoint being a change in mean HAM-D (Hamilton Depression Rating scale) scores. The results show that the addition of 5-HTP and creatine led to a mean 60% decrease in HAM-D scores.
Kious, B. M., Sabic, H., Sung, Y. H., Kondo, D. G., & Renshaw, P. (2017). An Open-Label Pilot Study of Combined Augmentation With Creatine Monohydrate and 5-Hydroxytryptophan for Selective Serotonin Reuptake Inhibitor- or Serotonin-Norepinephrine Reuptake Inhibitor-Resistant Depression in Adult Women. Journal of clinical psychopharmacology, 37(5), 578–583. https://doi.org/10.1097/JCP.0000000000000754