Vitamin B6 (Pyridoxine)
B vitamin for amino acid metabolism, neurotransmitter production, and immune function
Also known as
Definition
Vitamin B6 (an umbrella term for pyridoxine, pyridoxal, and pyridoxamine) is a water-soluble B vitamin whose active form pyridoxal-5-phosphate (PLP) serves as a cofactor for more than 100 enzymes in amino acid metabolism. PLP is essential for the synthesis of neurotransmitters (serotonin, dopamine, GABA) and hemoglobin, as well as for homocysteine conversion. High-dose supplementation can cause peripheral neuropathy.
| Parameter | Value |
|---|---|
| Unit | nmol/l (µg/l) |
| Reference Range | Plasma PLP: 20–150 nmol/l (~5–37 µg/l) |
Agree on your personal target range with your doctor.
Plasma PLP: 1 nmol/l = 0.247 µg/l. Reference ranges usually given as plasma PLP; US: 5–50 µg/l.
↓ What a low value means
B6 deficiency occurs with inadequate intake, chronic alcoholism, autoimmune disease, B6 antagonist medications (isoniazid, penicillamine), or increased demand (pregnancy). Symptoms: seborrheic dermatitis, glossitis, microcytic anemia, peripheral neuropathy, depression, elevated homocysteine, weakened immune defense.
↑ What a high value means
High-dose supplements can raise the level sharply. Chronically high B6 doses over months can cause sensory peripheral neuropathy with numbness and tingling in hands and feet (paradoxical toxicity). In 2023 EFSA set a tolerable upper intake level for adults that also applies to pregnant and breastfeeding women; it is based on the risk of peripheral neuropathy (EFSA overview of tolerable upper intake levels, version 11, August 2025). It is a safety limit, not an intake recommendation.
✓ What can influence this value
The vitamin B6 level is influenced by intake from food (e.g., poultry, tuna, salmon, potatoes, chickpeas, bananas, nuts), chronic alcohol consumption, autoimmune disease, B6 antagonists such as isoniazid and penicillamine, and increased demand in pregnancy. High-dose supplements can raise the level sharply and are associated with peripheral neuropathy. The active form pyridoxal-5-phosphate (P5P) does not need to be activated in the liver. The assessment is made by the treating doctor.
When to test
With neuropathy, depression, elevated homocysteine, PMS symptoms, chronic alcoholism, or certain medications (isoniazid, theophylline). Plasma PLP is the preferred marker over total B6.
Frequently asked questions
What is the relationship between vitamin B6 and PMS symptoms? +
Several studies show that B6 can reduce PMS symptoms such as mood swings, irritability, and anxiety, as B6 is required for serotonin and GABA synthesis. Evidence is moderate. The study doses were well above the EFSA tolerable upper intake level, and high doses carry a risk of neuropathy. Your doctor decides whether and how to treat.
What is the difference between pyridoxine and pyridoxal-5-phosphate (P5P)? +
Pyridoxine (pyridoxine HCl) must be converted in the liver to P5P, the active form. P5P itself does not need to be activated. Labs usually measure this active form as plasma PLP.
Can vitamin B6 cause neuropathy? +
Paradoxically yes: although B6 deficiency causes neuropathy, chronic overdosing over months can trigger sensory neuropathy. Symptoms are usually reversible after stopping. In 2023 EFSA set a tolerable upper intake level for adults, based on this risk of neuropathy.
What is the relationship between B6 and homocysteine? +
B6 is the cofactor of cystathionine beta-synthase, which converts homocysteine via the transsulfuration pathway to cystathionine. Together with B12 and folate, B6 forms the triad of B vitamins regulating homocysteine. Deficiency in any one of the three can cause elevated homocysteine.
Vitamin B6 (Pyridoxine)
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.