What biological systems are affected by Vitamin B6 (or B vitamins in general)?
The main effect, of vitamin B6, is neurological, becouse  moderate variations in the intracellular concentration of pyridoxal 5'-phosphate (PLP), the biologically active form of vitamin B6, can have pronounced modulatory effects on steroid-induced gene expression. So you cannot get the effect of stress hormones. 

Also neurological damage has been reported on doses from excess of 500 mg/d. Check this papers http://www.ncbi.nlm.nih.gov/pubmed/2661220 and http://www.fasebj.org/content/8/3/343.short