Short answer: magnesium bisglycinate and citrate are the best-absorbed of the widely available forms and the gentlest on the stomach. Magnesium oxide packs the most magnesium into the smallest tablet but is poorly absorbed and most likely to cause loose stools. If you are choosing between products and the elemental magnesium amount is similar, bisglycinate or citrate is usually the better buy.
The complication is that supplement labels rarely make this comparison easy. Here is how to read them.
Why the form matters at all
Magnesium is never sold as pure magnesium metal — it is always bound to something else, and that partner molecule determines how much of it your body absorbs and how your gut reacts. Two products can both say "magnesium 375 mg" on the front and deliver meaningfully different amounts into your bloodstream.
Under EU-authorised health claims, magnesium contributes to normal muscle function, normal psychological function, normal protein synthesis, normal energy-yielding metabolism, the maintenance of normal bones and teeth, electrolyte balance, and the reduction of tiredness and fatigue. Those claims apply to magnesium as a nutrient — no specific form is authorised to claim more than another.
The forms you will actually encounter
Magnesium bisglycinate (magnesium glycinate)
Magnesium bound to glycine, an amino acid. Well absorbed and the least likely of the common forms to cause digestive upset, which is why it is the usual choice for higher daily amounts or for people who have reacted badly to cheaper magnesium. The trade-off is bulk: because glycine is heavy relative to magnesium, you need more capsules for the same elemental dose, and the price per serving is higher.
Magnesium citrate
Magnesium bound to citric acid. Well absorbed, widely available, and cheaper than bisglycinate. It has a mild laxative effect at higher doses — which is a drawback if you are taking it for magnesium, and occasionally the point if you are not. A reasonable default for most people.
Magnesium oxide
The cheapest form and the highest magnesium content by weight — roughly 60% elemental magnesium, versus around 14% for bisglycinate. That looks attractive on a label. But absorption is poor, so a large share passes through unabsorbed, drawing water into the bowel on the way. Oxide is why so many people conclude that "magnesium upsets my stomach".
Magnesium malate
Bound to malic acid. Well absorbed, sometimes chosen for daytime use. The evidence base is thinner than for citrate or bisglycinate; there is no strong reason to prefer it over either, nor to avoid it.
Magnesium L-threonate
The newest and most expensive form, marketed for cognitive support on the basis that it crosses into the central nervous system more readily. Human research is early and limited. If you are buying magnesium for general nutritional intake, this is not where your money is best spent.
Magnesium chloride, sulfate, oils and sprays
Magnesium sulfate is Epsom salt, used in baths. Topical magnesium products are popular, but evidence that meaningful amounts absorb through intact skin is weak. Enjoy an Epsom bath for the bath; do not count it as your magnesium intake.
How to compare two products properly
Ignore the number on the front of the pack and find these three things on the back:
- Elemental magnesium per daily serving. This is the figure that matters, usually shown with a %NRV or %DRI. "Magnesium citrate 500 mg" is not the same as "500 mg magnesium" — the first is the whole compound.
- How many units make up that serving. A product needing three capsules for 144 mg is a different proposition from one needing one tablet for 375 mg, both in cost and in convenience.
- Which form, and whether there are several. Blends are common. A product listing oxide first is mostly oxide, whatever else appears further down.
Then divide the price by the number of daily servings, not by the number of capsules in the tub.
How much, and when
Adult reference intakes in the EU sit around 375 mg of elemental magnesium daily from all sources — food included. Most people get a substantial share from diet: wholegrains, legumes, nuts, seeds and leafy greens are all good sources. Supplements make up a shortfall rather than replacing that.
On timing: labels vary, and the label is the authority. Many people take magnesium in the evening because they find it settling, and magnesium's authorised claim for normal psychological function makes it a common part of evening routines. There is no requirement that it be taken at night. Splitting the dose across the day often reduces digestive complaints.
The signal you have taken too much
Loose stools. That is the practical upper limit for most people, and it arrives well before anything more serious. Supplemental magnesium is generally well tolerated in healthy adults, but it is worth knowing that impaired kidney function changes this picture entirely — magnesium is cleared by the kidneys, and people with reduced kidney function can accumulate it. If that applies to you, do not supplement magnesium without medical advice.
Interactions worth knowing
Magnesium can reduce absorption of certain antibiotics (tetracyclines, quinolones), bisphosphonates and levothyroxine. The usual advice is to separate them by a few hours, but check with your pharmacist about your specific medication rather than guessing. If you take a diuretic, a proton pump inhibitor long-term, or medication for a heart condition, raise magnesium supplementation with your doctor — all three can affect magnesium status.
What we stock, and where each fits
Across our natural supplements range you will find most of these forms:
- Bisglycinate / glycinate — Holistic Magnesium Bisglycinate 100 mg, Elexir Magnesium Bisglycinate, Great Earth Magnesium Glycinate. Best starting point if cheaper magnesium has upset your stomach.
- Citrate and blends — Elexir Magnesium 375 mg, Great Earth Magnesium 375 mg, Skip Magnesium 250 mg, New Nordic Magic Magnesium Citrate. Sensible everyday options.
- L-threonate — Holistic Brain-Mag, for anyone specifically interested in that research area.
- Powders, drops and sprays — Great Earth Magnesium Powder, Holistic Magnesium Drops and Magnesium Spray, for people who dislike tablets.
- Epsom salt — Elexir and Elexir Pharma 1 kg bath salts, in our bath salts collection.
The honest summary
Form matters less than people on the internet suggest, and less than dose and consistency. If you are currently taking magnesium oxide and tolerating it fine, you are probably absorbing enough. If magnesium has ever "disagreed" with you, switch to bisglycinate before concluding magnesium is not for you. And if you are buying magnesium hoping to fix persistent fatigue or poor sleep, it is worth getting bloodwork first — iron, vitamin D, B12 and thyroid function are more common culprits, and a supplement cannot substitute for finding the actual cause.
Food supplements should not replace a varied, balanced diet and a healthy lifestyle. Always read the product label and follow the stated directions. Seek advice from a doctor or pharmacist before supplementing if you are pregnant or breastfeeding, take prescription medication, or manage a medical condition — particularly reduced kidney function.
