I came across this recent article that seems to indicate that it is specifically the pyridoxine form of B6 that can kill cells. (There are other forms, such as pyridoxal, pyridoxal-5-phosphate and pyridoxamine that DON'T injure cells.)

You can buy pyridoxal-5-phosphate and get a prescription for pyridoxamine, but the form most readily available in most supplements is pyridoxine!

Toxicol In Vitro
. 2017 Oct;44:206-212. doi: 10.1016/j.tiv.2017.07.009. Epub 2017 Jul 14.
The Vitamin B6 Paradox: Supplementation With High Concentrations of Pyridoxine Leads to Decreased Vitamin B6 Function

Misha F Vrolijk 1 , Antoon Opperhuizen 2 , Eugène H J M Jansen 3 , Geja J Hageman 4 , Aalt Bast 4 , Guido R M M Haenen 4
Affiliations expand
PMID: 28716455 DOI: 10.1016/j.tiv.2017.07.009
Abstract

Vitamin B6 is a water-soluble vitamin that functions as a coenzyme in many reactions involved in amino acid, carbohydrates and lipid metabolism. Since 2014, >50 cases of sensory neuronal pain due to vitamin B6 supplementation were reported. Up to now, the mechanism of this toxicity is enigmatic and the contribution of the various B6 vitamers to this toxicity is largely unknown. In the present study, the neurotoxicity of the different forms of vitamin B6 is tested on SHSY5Y and CaCo-2 cells. Cells were exposed to pyridoxine, pyridoxamine, pyridoxal, pyridoxal-5-phosphate or pyridoxamine-5-phosphate for 24h, after which cell viability was measured using the MTT assay. The expression of Bax and caspase-8 was tested after the 24h exposure. The effect of the vitamers on two pyridoxal-5-phosphate dependent enzymes was also tested. Pyridoxine induced cell death in a concentration-dependent way in SHSY5Y cells. The other vitamers did not affect cell viability. Pyridoxine significantly increased the expression of Bax and caspase-8. Moreover, both pyridoxal-5-phosphate dependent enzymes were inhibited by pyridoxine. In conclusion, the present study indicates that the neuropathy observed after taking a relatively high dose of vitamin B6 supplements is due to pyridoxine. The inactive form pyridoxine competitively inhibits the active pyridoxal-5'-phosphate. Consequently, symptoms of vitamin B6 supplementation are similar to those of vitamin B6 deficiency.

Keywords: Neuropathy; Neurotoxic; Pyridoxine; Supplements; Vitamin B6.

Copyright © 2017 Elsevier Ltd. All rights reserved.