Magnesium Has 8 Forms — Only 2 Actually Work. Here's Which to Buy.

Published March 2026 • 4 min read

Key Takeaways

75% of adults are magnesium deficient — and standard blood tests don't catch it
Magnesium glycinate (for sleep and calm) and magnesium L-threonate (for brain health) are the only two forms worth buying
Magnesium oxide — the cheapest and most common form — has only 4% bioavailability

You're probably taking the wrong magnesium. I know because I did for years. The bottle said "magnesium" and I assumed that was good enough. It wasn't. And the difference between the right form and the wrong one isn't subtle — it's the difference between a supplement that works and expensive urine.

The 4% Scandal

Walk into any pharmacy and grab the cheapest magnesium on the shelf. Flip it over. It almost certainly says magnesium oxide. This is the form that roughly 70% of all magnesium supplements use because it's dirt cheap to manufacture.

The bioavailability of magnesium oxide is approximately 4%. That means for every 400mg capsule, your body absorbs about 16mg. The rest? Straight through your digestive system. The most common side effect of magnesium oxide isn't relaxation or better sleep. It's diarrhoea. That should tell you everything you need to know about how much your body is actually using.

The Two That Actually Work

Magnesium glycinate is the gold standard for sleep, anxiety, and muscle relaxation. The glycine amino acid it's bound to is itself calming — it activates GABA receptors in your brain. Bioavailability sits around 80%. It doesn't cause GI distress. Take 200-400mg about an hour before bed, and the difference in sleep quality is noticeable within 3-5 days. Not subtle. Noticeable.

Magnesium L-threonate is the only form shown to cross the blood-brain barrier in meaningful amounts. Developed at MIT, it was specifically engineered to increase brain magnesium levels. A 2010 study in the journal Neuron showed it enhanced synaptic density and improved both short-term and long-term memory in animal models. Human trials have since confirmed cognitive improvements, particularly in older adults. This is the form for brain health. Period.

The Other Six Forms You Can Ignore

Magnesium citrate is decent for general supplementation and has good bioavailability, but it's mainly a laxative. Magnesium malate has some evidence for muscle pain and fatigue. Magnesium taurate is sometimes recommended for heart health. The rest — sulfate, chloride, orotate — are either poorly absorbed, poorly studied, or both.

If you're only going to buy one, get glycinate for sleep. If you're over 40 and care about your brain, add L-threonate. Everything else is marketing.

Why Your Blood Test Says You're Fine

Here's the frustrating part. Standard serum magnesium tests are nearly useless. Only 1% of your body's magnesium is in your blood. The other 99% is in your bones, muscles, and cells. Your body will strip magnesium from your bones to keep blood levels stable. So your blood test says "normal" while your cells are starving.

An RBC (red blood cell) magnesium test is more accurate but rarely ordered. The real indicator is symptoms: poor sleep, muscle cramps, anxiety, brain fog, irregular heartbeat. Sound familiar? That's not ageing. That's magnesium deficiency masquerading as getting old.

Get Longevity Insights Delivered

Science-backed health tips delivered every week.

Medical Disclaimer: This content is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider before starting any new supplement or making changes to your health regimen.