How to Compare Magnesium Supplement Labels

Magnesium shelves are crowded: glycinate, citrate, oxide, malate, “complex”, “high strength”. The numbers on the front don’t always measure the same thing either. This guide shows how to line products up fairly using what the label says.

It doesn’t pick a best product, and it doesn’t suggest how much magnesium anyone should take. Choosing a product properly would mean testing real products, and an amount that suits one person may not suit another.

The criteria at a glance

1. Magnesium per serving, not compound weight

The FDA requires the Supplement Facts panel to state the weight of the mineral, not the weight of the compound it comes in. That makes the panel figure the one to compare.

Numbers on the front of a bottle are worth checking against the panel. A figure such as “Magnesium glycinate 1,000 mg” could refer to the weight of the compound or to the magnesium it supplies, and the two are very different. Based on its molecular weight in PubChem, magnesium bisglycinate is about 14% magnesium, so 1,000 mg of the compound would supply roughly 140 mg of magnesium, or less if the ingredient contains water. The Supplement Facts panel shows the magnesium itself, so when the front and the panel seem to disagree, go by the panel.

2. The form

The form is often named in brackets next to the magnesium amount, for example “Magnesium (as magnesium bisglycinate)”. If it isn’t named on the panel, FDA rules mean it has to appear in the ingredient list instead.

These percentages apply to the pure chemical forms named. Commercial ingredients can differ, for example because some contain water (hydrates) or are blended with other forms, so take the actual amount of magnesium from the Supplement Facts panel.

A 2021 systematic review of 14 studies concluded that inorganic forms such as oxide appear to be less bioavailable than organic forms such as citrate, and that the percentage absorbed depends on the dose. The same review found that all the forms studied could maintain normal magnesium levels in healthy people without a deficit. Our article on magnesium glycinate vs citrate looks at those two forms in more detail.

Some products combine several forms. The label should name each source, which helps if one of them is the reason you’re buying.

3. Serving size

Forms with a low share of magnesium need more compound per milligram of magnesium, so a product may spread one serving over several capsules. A bottle of 120 capsules at four per serving lasts 30 servings, not 120. Our guide to reading a Supplement Facts label covers serving sizes and the rest of the panel.

4. Other ingredients

The ingredient statement lists everything that isn’t a dietary ingredient: the capsule shell (gelatin or plant-based), fillers, flow agents, and in powders and gummies, flavourings and sweeteners. It’s the place to check if you avoid particular ingredients.

5. Independent testing

A manufacturer’s own “certified” or “verified” wording doesn’t necessarily guarantee quality. If a product shows a seal from an independent testing organisation, check the organisation’s own website for what was tested and whether that product is listed.

Testing matters because products that look alike on the label can behave differently. In a 2019 study, 15 commercially available magnesium products showed very wide differences in dissolution and bioaccessibility in laboratory models. Two formulations with contrasting laboratory results were then tested in 30 people, and the one that performed poorly in the laboratory also showed lower bioavailability in the body. No label shows that kind of difference.

6. Price per serving and per 100 mg of magnesium

Price per bottle hides big differences in strength and serving size. Two made-up products:

Product B is cheaper per serving and slightly cheaper per 100 mg of magnesium, but it provides half as much per serving. Neither figure tells you which product is better; together, they make the comparison fair.

Amounts in context

The NHS gives these reference figures for adults aged 19 to 64: 300 mg of magnesium a day for men and 270 mg for women. It says you should be able to get all the magnesium you need from a varied and balanced diet. It also reports UK government advice that 400 mg or less a day from supplements is unlikely to cause harm, and notes that more than 400 mg taken for a short time can cause diarrhoea. Other countries’ health authorities publish their own figures.

Some magnesium citrate products are also used as saline laxatives, as MedlinePlus explains. If you take regular medication or have a health condition, a pharmacist or doctor can help you decide whether a magnesium supplement makes sense for you.