The Maths of a Vitamin B6 Overdose

16 Nov, 2022 • 6 min read

Setting the scene

This year I've spent 8 months looking for answers to some bizarre, elusive and unnerving physical symptoms.

The first diagnosis was stress and anxiety — which was quite true — but that couldn't explain (among other symptoms) why each morning I couldn't feel where my hands were. It's not just that they were numb — or on fire, depending on the morning — my proprioception was completely busted.

After months of blood tests, therapy, heart tests, Parkinsons tests, neuro-physiotherapy, nerve conduction studies and MRIs, I was sitting in a hotel room after a work on-site when I recieved an email from my mum.

By sheer coincidence, someone in her office had sent her this update from the TGA, which she passed on to me because the description of the peripheral neuropathy symptoms matched many of the symptoms I'd been experiencing.

Adverse event reports submitted to the TGA suggest there is a lack of awareness that vitamin B6, which is present in many multivitamin and mineral supplements, can cause peripheral neuropathy. In response, the TGA has strengthened labelling requirements so products containing daily doses over 10mg of vitamin B6 must carry a warning about peripheral neuropathy.

After a bit of Googling I found the Wikipedia page for Megavitamin-B6 Syndrome, and sat very still (difficult when your fingers twitch uncontrollably) mentally ticking off every single symptom listed.

But I've never taken Vitamin B6 supplements.

Or at least, I thought I hadn't.

After a quick rummage in my luggage, I discovered the Magnesium supplement I'd been taking every day since March also contained a 41.1mg dose of Pyridoxine hydrochloride (i.e. Vitamin B6). The recommended daily dietary intake of Vitamin B6 for someone my age is about 1.1mg.

I'd been poisoning myself with a supplement I didn't even know contained B6 because I didn't read the fine print.

Thanks Blackmore's.

So naturally I stopped taking the Magnesium immediately. From the Wikipedia page, the half-life of B6 in the body is 33 days. And there's this delighful footnote:

In the immediate 2–6 weeks following discontinuation of vitamin B6, patients may experience a symptom progression before gradual improvement begins. This is known as coasting and is encountered in other toxic neuropathies.

As I write this, it's Day 36, and my symptoms did indeed progress for a while, but have started to ease noticeably. They're not completely gone though, and won't be for a few more months.

Math overdose

Half-life is not elimination, so I thought I'd do some quick maths to figure out:

How much Vitamin B6 I had in my body the day I stopped taking the Magnesium
How much Vitamin B6 I should have had in my body that day
How much Vitamin B6 I had in my body on Day 33 (one half-life later)

Assumptions:

Every day I don't take a supplement, I'm ingesting the RDI of 1.1mg Vitamin B6.
Every day I do take the supplement, I'm ingesting the RDI plus 41.1mg from the Magnesium tablet (42.2mg in total).
I had a negligible amount of Vitamin B6 in my body on March 1 (close enough).
The half-life of B6 in the body is 25-33 days. Let's assume a worst-case 33 days.

Facts:

I started the Magnesium supplements on March 2
I stopped taking them on October 10 (222 days later)
A compound's half-life is the time it takes for half the mass of the compound to be eliminated from the system

Working:

The half-life equation is a decreasing exponentional:

The Half-Life Equation

As each daily dose of B6 is the same (42.2mg), we can use the following summation to find out how much is in my body on the day I dicovered I had been poisoning myself. The summation is "counting backwards" from the previous day's dose:

Summation for how much Vitamin B6 was in my body after supplemtnation for 222 days

The result of this summation is approximately 1970mg or 1.97 grams.

The amount of B6 I should have had in my body is the same summation, but with a healthy daily dose of 1.1mg:

Summation for how much Vitamin B6 should have been my body with no supplementation for 222 days

The result is approximately 51mg, and is a good approximation for the healthy steady-state level, given the small numbers involved.

So I was walking around with about 38x the recommended healthy amount of Vitamin B6 in my body.

After 33 days of no supplementation, we can determine the amount of B6 left in my body with this summation:

Summation for how much Vitamin B6 was still in my body after 33 days without eupplementation, following 222 days of supplementation

Which is approximately 1010mg. Just over 1g. That's still 20x what I should have floating around.

After another 33 days: 531mg (10x)
After another 33 days: 292mg (5.7x)
After another 33 days: 172mg (3x)
After another 33 days: 112mg (2.2x)
After another 33 days: 82mg (1.6x)
After another 33 days: 69mg (1.4x)

The exponential decline means it will be a long time before I actually get back to the 51mg mark exactly, but it's safe to assume after 6 months I'll be back under 100mg, which is well and truly safe.

Final note

Not all Magnesium supplements have Vitamin B6 in them, I just got unlucky. But I've sworn off all supplements for at least 6 months anyway. My occasional multivitamin was also providing a bit of a bump (~3mg), and while the Vitamin D capsules are pure, I'm putting them on hold for a bit too.