Elevated vitamin B6 whole blood levels in Australian patients

To the Editor: Paradoxically, both vitamin B6 deficiency and excess are associated with peripheral neuropathy. Increasingly, patients are presenting with peripheral neuropathy associated with elevated vitamin B6 whole blood levels, as previously presented by our laboratory.1

With a societal push towards wellness, there has been a greater uptake of over‐the‐counter vitamins in recent years. Magnesium is taken by patients for muscle, nerve and digestive issues, as well as for stress, anxiety and overall wellness. For some of these listed indications, the evidence is dubious. Vitamin B6 (as pyridoxine hydrochloride) is common among magnesium supplements based on the theory that the cellular uptake of magnesium is enhanced.2 These supplements are readily available in pharmacies, supermarkets and online, and contain up to 60mg of vitamin B6 per tablet.

When purchasing magnesium supplements, patients may be unaware that they are also getting vitamin B6 and the quantity included could be hazardous to health. A patient taking two magnesium tablets per day could be unintentionally consuming 120mg of vitamin B6, now considered a large dose. This is far greater than the recommended dietary intake of vitamin B6 for adults in Australia and New Zealand of 1.3–2.0mg/day with a maximum of 50mg/day.3

The pharmacokinetics of pyridoxine hydrochloride are complex. Despite being water soluble, it has a half‐life of up to 30 days. Repetitive small doses taken over a period of months can accumulate, resulting in high blood concentrations. Vitamin B6 is associated with peripheral neuropathy when whole blood levels exceed 250nmol/L.4 Our laboratory recently presented a study showing that about 39% of our patients had elevated vitamin B6 whole blood levels (>190mol/L, the upper limit of our reference interval). The study included data from 88655 samples.1

A recent regulatory change from the Therapeutic Goods Administration mandated that products containing vitamin B6 in daily doses above 10mg now require a label warning.5 This previously applied to products containing over 50mg per daily dose. The Therapeutic Goods Administration has also reduced the maximum permitted daily dose from 200mg to 100mg for adults. It also noted cases where peripheral neuropathy developed in patients taking less than 50mg daily.5 This change applies to products produced after 1 March 2023 and therefore does not apply to products purchased before this date.

In addition to being present in magnesium supplements, vitamin B6 (as pyridoxine hydrochloride) is contained in many multivitamins and energy drinks. Community awareness is key.