The upper limit is lower than the recommended amount. Understanding why not only resolves the contradiction — it tells you how to read a magnesium label.
The short answer. The Tolerable Upper Intake Level for magnesium is 350 mg a day for everyone aged nine and up. That figure counts only magnesium from supplements and medications — not the magnesium in your food, which is why it can sit below the recommended daily intake without contradicting it. And it is not a poison threshold. The first thing that happens above it is usually loose stools, not harm.
What the upper limit actually is
The Food and Nutrition Board sets a Tolerable Upper Intake Level, or UL, for supplemental magnesium:
| Age | Upper limit |
|---|---|
| 0–6 months | None established |
| 7–12 months | None established |
| 1–3 years | 65 mg |
| 4–8 years | 110 mg |
| 9–13 years | 350 mg |
| 14–18 years | 350 mg |
| 19 years and older | 350 mg |
| Pregnancy and lactation | 350 mg |
The figure stops climbing at nine years old. A thirteen-year-old and a seventy-year-old have the same 350 mg ceiling.
Why is the upper limit lower than the recommended amount?
This is the part that confuses almost everyone, and it looks like an error until you see what each number is counting.
The Recommended Dietary Allowance for magnesium is 400–420 mg a day for adult men and 310–320 mg for adult women. So the recommendation for a man is 420 mg, and the ceiling is 350 mg. Eat what you are told to eat and you have apparently exceeded the limit.
You have not, because the two numbers count different things.
The RDA counts everything — food, drinks, supplements and medications together.
The UL counts only supplements and medications. Magnesium that occurs naturally in food and drink is not counted at all.
The distinction exists because of what the upper limit was built from. The adverse effects used to set it were the ones associated with magnesium taken as a supplement or a medication — diarrhea from high supplemental doses above all. Magnesium naturally present in food was not part of that picture, so it is not part of the number.
The practical version: a man taking in 420 mg a day, of which 220 mg comes from food and 200 mg from a supplement, is at his recommended intake and well under the ceiling. Both numbers are satisfied at the same time.
The number on the bottle is not the number to compare
Before you check a supplement against 350 mg, check which number you are holding.
A bottle that says "Magnesium Glycinate 1,000 mg" on the front is describing the weight of the compound, not the magnesium in it. The magnesium itself — the elemental figure — is a fraction of that, and it is the figure in the Supplement Facts panel next to a % Daily Value. Compare the front number against the upper limit and you will frighten yourself over a dose that may not be close to it. This is the most useful thing to understand about a magnesium label, and it is the source of most of the confusion about magnesium doses.
What happens first if you take too much
Not toxicity. Diarrhea.
High doses of magnesium from supplements or medications often produce diarrhea, sometimes with nausea and abdominal cramping. The mechanism is straightforward: the magnesium that is not absorbed stays in the gut, where it draws water in by osmosis and speeds things along. It is the same effect that makes magnesium salts useful as laxatives — that is not a side effect of a laxative, it is how the laxative works.
The forms most commonly reported to cause it are magnesium carbonate, chloride, gluconate and oxide.
This is worth understanding rather than fearing, because it is self-limiting and it is informative. The dose that gives you loose stools is telling you that a meaningful share of it did not get absorbed.
How far is a normal supplement from a dose that causes problems?
Far enough that the numbers are worth seeing.
Magnesium toxicity is a real condition, and the cases in the literature involve magnesium-containing laxatives and antacids — typically at more than 5,000 mg of magnesium a day. That is not a supplement dose. It is what comes from taking a product designed to move your bowels, in quantity, over time.
For scale: one over-the-counter laxative delivers 500 mg of elemental magnesium per tablespoon, and its own directions allow up to four tablespoons a day for adults. That is 2,000 mg — roughly six times the upper limit — from a product sold without a prescription. Some of it is never absorbed, precisely because of the laxative effect.
A 200 mg supplemental dose is below the adult upper limit of 350 mg a day, and far below the very large magnesium exposures reported in cases of toxicity.
Four bars on one linear scale, where 5,000 mg is the full width.
Too much magnesium from food does not pose this risk in healthy people. The kidneys clear the excess.
Which is why the kidneys are the whole story
Everything above assumes kidneys that work.
The risk of magnesium toxicity rises when kidney function is impaired or has failed, because the ability to remove the excess is reduced or lost. That single fact does more to determine whether magnesium is safe for a given person than the dose on the label does.
If you have reduced kidney function or kidney failure, talk to your healthcare professional before taking a magnesium supplement at all. The same goes if you take a medication that changes how much magnesium your body keeps — some medications do exactly that.
What this does not tell you
- It does not tell you that 350 mg is a danger line. It is a level below which adverse effects are unlikely, not a point at which harm begins.
- It does not tell you that magnesium from food should be limited. The upper limit deliberately excludes it.
- It does not tell you the right amount for you. Reference intakes describe populations, not individuals.
- It does not account for magnesium in medications you may already take. Some antacids and laxatives contain a great deal of it.
- It does not apply in the same way if your kidneys do not clear magnesium normally.
Before you act on any of this
This article is general education, not medical advice. If you have kidney disease, take prescription medication, or are unsure how much magnesium you are already getting, ask a pharmacist or your prescriber rather than working it out from a web page.
Sources
- Office of Dietary Supplements, National Institutes of Health. Magnesium — Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/
- Fine KD, Santa Ana CA, Porter JL, Fordtran JS. Intestinal absorption of magnesium from food and supplements. J Clin Invest. 1991;88(2):396–402. https://pubmed.ncbi.nlm.nih.gov/1864954/