A 2004 study  analyzed 140 subjects, of whom 18 (13%) had depressive symptoms, and found that the lower the blood concentration of active B6(P5P), the more depressive symptoms, and the study demonstrated a link between blood concentration of P5P and depression. A 2015 Canadian study  followed 170 subjects for 3 years and found that those with the most B6 in their diet plus nutritional supplements had a 43% lower risk of developing depression. Although the study was an association study, the risk analysis took into account subjects' health status and other humanistic factors.
Pyridoxal-5'-Phosphate (PLP or P-5'-P) is the main metabollically active conzyme form of vitamin B-6, that's why I chose it. It is a bit pricey but worth it. I need it mainly to help with fat metabolism, for a good immune system and for good brain function. So far I can't talk about positive effects, but I can say that I haven't noticed any bad ones so far.