The dorsal root ganglia, however, are located outside of the blood-brain barrier making them more susceptible.Pyridoxine is converted to pyridoxal phosphate via two enzymes, pyridoxal kinase and pyridoxine 5′-phosphate oxidase. High levels of pyridoxine can inhibit these enzymes. As pyridoxal phosphate is the active form of vitamin B6, this saturation of pyridoxine could mimic a deficiency of vitamin B6. Tolerable upper limits
Several government agencies have reviewed the data on vitamin B6 supplementation and produced consumption upper limits with the desired goal to prevent sensory neuropathy from excessive amounts. Each agency developed its own criteria for usable studies in relation to tolerable upper limits, and as such the recommendations vary by agency. Between agencies, current tolerable upper limit guidelines vary from 10 mg per day to 100 mg per day. Reviews of vitamin B6 related neuropathy cautioned that supplementation at doses greater than 50 mg per day for extended periods of time may be harmful and should be discouraged. In 2008, the Australian Complementary Medicines Evaluation Committee recommended warning statements appear on products containing daily doses of 50 mg or more vitamin B6 to avoid toxicity.The relationship between the amount of vitamin B6 consumed, and serum the levels of those who consume it, varies between individuals. Some people may have high serum concentrations without symptoms of neuropathy. It is not known if inhalation of vitamin B6 while, for example, working with animal feed containing vitamin B6 is safe.
Megavitamin-B6 syndrome is a collection of symptoms that can result from chronic supplementation, or acute overdose, of vitamin B6. While it is also known as hypervitaminosis B6, vitamin B6 toxicity and vitamin B6 excess, megavitamin-b6 syndrome is the name used in the ICD-10. Signs and symptoms
The predominant symptom is peripheral sensory neuropathy that is experienced as numbness, pins-and-needles and burning sensations (paresthesia) in a patients limbs on both sides of their body. Patients may experience unsteadiness of gait, incoordination (ataxia), involuntary muscle movements (choreoathetosis) the sensation of an electric zap in their bodies (Lhermittes sign), a heightened sensitivity to sense stimuli including photosensitivity (hyperesthesia), impaired skin sensation (hypoesthesia), numbness around the mouth, and gastrointestinal symptoms such as nausea and heartburn. The ability to sense vibrations and to sense ones position are diminished to a greater degree than pain or temperature. Skin lesions have also been reported. Megavitamin-B6 syndrome may also contribute to burning mouth syndrome. Potential psychiatric symptoms range from anxiety, depression, agitation, and cognitive deficits to psychosis.Symptom severity appears to be dose-dependent (higher doses cause more severe symptoms) and the duration of supplementation with vitamin B6 before onset of systems appears to be inversely proportional to the amount taken daily (the smaller the daily dosage, the longer it will take for symptoms to develop). It is also possible that some individuals are more susceptible to the toxic effects of vitamin B6 than others.