Megavitamin-B6 syndrome
Vitamin B6 excess causes peripheral neuropathy, often reversible upon cessation.
Megavitamin-B6 syndrome, also called hypervitaminosis B6, vitamin B6 toxicity, or vitamin B6 excess, refers to a set of symptoms caused by taking too much vitamin B6 over time or in a single large dose. The condition is listed under this name in the ICD-10. It mainly affects the peripheral nerves, and while symptoms often go away after stopping the supplement, this is not guaranteed. When recovery does occur, it usually happens within six months, though some symptoms may briefly get worse right after stopping—a phenomenon known as "coasting." Diagnosis involves blood tests showing high vitamin B6 levels, along with nerve conduction studies and other neurological exams. The syndrome demonstrates that a substance safe at recommended doses can become harmful when taken in excess.
The main symptom is peripheral sensory neuropathy, causing numbness, tingling, and burning sensations on both sides of the body. Patients may also experience unsteady walking, poor coordination, involuntary muscle movements, a sensation like an electric shock in the body (Lhermitte's sign), increased sensitivity to stimuli such as light, reduced skin sensation, numbness around the mouth, and gastrointestinal issues like nausea and heartburn. The ability to feel vibrations and body position is more affected than pain or temperature sense. Skin lesions have been reported, and the syndrome may contribute to burning mouth syndrome. Psychiatric effects can include anxiety, depression, agitation, cognitive problems, and even psychosis.
Symptom severity depends on the dose—higher doses cause worse symptoms. The time it takes for symptoms to appear is inversely related to the daily amount: smaller doses take longer to cause problems. Some people may be more susceptible than others. Cases have been reported with doses as low as 24 mg per day. The form of vitamin B6 in supplements may also matter; some suggest using pyridoxal or pyridoxal phosphate instead of pyridoxine to reduce toxicity risk. However, a tissue culture study found that all B6 forms that can be turned into active coenzymes (pyridoxal, pyridoxine, and pyridoxamine) were similarly neurotoxic at equal concentrations. In living organisms, supplementing with pyridoxal or pyridoxal phosphate raises pyridoxine levels, indicating that the body can convert one form into another.
Quick Facts
- Synonym
- Vitamin B / 6 / Excess, Hypervitaminosis B / 6 / , Vitamin B / 6 / Toxicity
- Specialty
- Neurology, toxicology
- Symptoms
- Peripheral sensory neuropathy
- Onset
- Gradual onset with slow progression, in the usual case of chronic vitamin B / 6 / supplementation.
- Duration
- Usually, but not always, resolves within 6 months from cessation of vitamin B / 6 / .
- Risks
- Impaired kidney function, parenteral nutrition
- Causes
- Chronic vitamin B / 6 / supplementation, or acute parenteral or oral over‐dosages of vitamin B / 6 / .
- Diagnosis
- Serum testing for elevated levels of vitamin B / 6 / , testing of tendon reflexes, nerve conduction studies and electrodiagnostic testing.
- Differential
- Progressive mixed sensory or sensorimotor polyneuropathy of undetermined etiology.
- Treatment
- Cessation of vitamin B / 6 / supplementation.
- Prognosis
- Symptom progression for 2-6 weeks following cessation of vitamin B / 6 / , followed by gradual improvement.
Facts from the source article.
Lore & Background
The syndrome is also known as hypervitaminosis B6, vitamin B6 toxicity, and vitamin B6 excess. The predominant symptom is peripheral sensory neuropathy, experienced as numbness, pins-and-needles, and burning sensations in both sides of the body. Other symptoms include unsteadiness of gait, incoordination, involuntary muscle movements, Lhermitte's sign, heightened sensitivity to stimuli, impaired skin sensation, numbness around the mouth, gastrointestinal symptoms, skin lesions, and potential psychiatric symptoms ranging from anxiety to psychosis. Symptom severity is dose-dependent, and the duration before onset is inversely proportional to daily dosage. Some individuals may be more susceptible than others. The condition shows that a substance safe at recommended dosages can cause serious harm when taken in excess.
Reader's Guide
Megavitamin-B6 syndrome is classified as a sensory ganglionopathy due to degeneration of dorsal root ganglion axons and cell bodies, also affecting the trigeminal ganglia. Diagnosis involves serum tests for elevated vitamin B6, nerve conduction studies, and other neurodiagnostic evaluations. The clinical hallmark is ataxia due to sensory polyneuropathy. Treatment primarily consists of stopping supplemental vitamin B6; physical therapy and medications such as amitriptyline may help. Prognosis is generally good, with symptoms usually resolving within six months, though extremely high doses can cause irreversible symptoms. The syndrome has led government agencies to set tolerable upper limits varying from 10 mg to 100 mg per day. Reviews caution against supplementation above 50 mg per day for extended periods. The condition underscores the importance of dosage limits for water-soluble vitamins.
Did You Know?
- Megavitamin-B6 syndrome has been reported at doses as low as 24 mg per day.
- Symptoms may briefly intensify after stopping vitamin B6, a phenomenon called 'coasting'.
- The dorsal root ganglia are susceptible because they lie outside the blood-brain barrier.
- High doses of vitamin B6 are used to treat isoniazid overdose and gyromitra mushroom poisoning.
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