The study claims that multi-year adminstration of vitamin B6 completely "normalized" ADHD symptoms. It was a human study, so it should have even more weight / relevance.
http://www.ncbi.nlm.nih.gov/pubmed/24321736
"...The disturbed activity of PLP dependent enzymes apparently forms those profound disturbances of neurotransmitter systems, which are inherent in ADHD: low concentrations of monoamines and disordered amino acid metabolism. If vitamin B6 disorders are the core biochemical disturbances inherent in ADHD, then the long-term pyridoxine treatment is pathogenetically based replacement therapy of the disease. According to our data, multi-year pyridoxine treatment normalizes completely the pattern of ADHD behavior, without causing any serious side effects."
http://www.ncbi.nlm.nih.gov/pubmed/24321736
"...The disturbed activity of PLP dependent enzymes apparently forms those profound disturbances of neurotransmitter systems, which are inherent in ADHD: low concentrations of monoamines and disordered amino acid metabolism. If vitamin B6 disorders are the core biochemical disturbances inherent in ADHD, then the long-term pyridoxine treatment is pathogenetically based replacement therapy of the disease. According to our data, multi-year pyridoxine treatment normalizes completely the pattern of ADHD behavior, without causing any serious side effects."