Independent supplement ratingsUS and Canada · Updated weekly
A ratings desk for a category that has spent thirty years grading its own homework.
A ratings desk for a category that has spent thirty years grading its own homework.
Oxide · 200 tablets · one a day
250mg elemental / 200–400mg studied
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The cheapest magnesium in this ranking by a distance — about five cents a serving — and the label is honest: 250 mg of elemental magnesium per tablet, squarely inside the 200–400 mg range trials use. The catch is the form. Magnesium oxide is the cheapest to manufacture and the worst absorbed of the common forms; in a comparison of US commercial preparations its fractional absorption came out at about 4%. So the number on the label is true, and the amount that reaches you is not the same number.
One tablet, 250 mg magnesium as magnesium oxide, no proprietary blend, nothing hidden — a clearer label than most of this category manages. We take the figures from the product's entry in the NIH Dietary Supplement Label Database rather than from marketing copy. Where it loses points is absorption, not disclosure: a well-absorbed 200 mg of glycinate does more than a poorly-absorbed 250 mg of oxide, which is why the score discounts what the panel promises.
Two honest cases. If you want the cheapest possible magnesium top-up and your gut tolerates it, this is unbeatable value. And oxide's poor absorption is a feature rather than a flaw if constipation is what you're treating — that's a legitimate use of it. For sleep, cramps or a real shortfall, spend the extra few cents on a glycinate. Why the form eats the dose →
Reviewed and fact-checked by the NFS editorial team · Last updated 12 Jul 2026