Taking too much magnesium from supplements typically causes diarrhea, nausea, and cramping first. At higher levels, it can lead to dangerously low blood pressure, breathing difficulties, and in extreme cases, cardiac arrest. The tolerable upper intake level for supplemental magnesium is 350 mg per day for adults. That limit applies only to magnesium from supplements and medications, not from food.
The First Sign: Digestive Problems
The most common result of taking too much magnesium is loose stools or outright diarrhea. Most magnesium supplements are salts, meaning the magnesium is bound to another molecule to help with absorption. When your body can’t absorb all the magnesium you’ve taken, the excess stays in your gut and pulls water in with it. That extra water loosens your stool and speeds up bowel movements.
Some forms are more likely to cause this than others. Magnesium citrate dissolves easily and is actually used as a laxative before medical procedures like colonoscopies. Magnesium oxide, commonly sold as an antacid, also has strong laxative effects because it’s poorly absorbed. If you’re taking either of these forms at moderate to high doses, diarrhea is almost expected. Three or more loose or watery bowel movements a day is the threshold where it becomes a clinical concern, since ongoing diarrhea can lead to dehydration and electrolyte imbalances.
Why 350 mg Is the Limit
The National Institutes of Health sets the tolerable upper intake level for supplemental magnesium at 350 mg per day for anyone age 9 and older, including pregnant and lactating women. For children ages 4 to 8, the limit is 110 mg. For toddlers ages 1 to 3, it’s 65 mg.
An important nuance: this limit only covers magnesium from supplements and medications. It doesn’t include the magnesium you get from food. That’s why the upper limit can actually appear lower than the recommended daily allowance, which counts all dietary sources combined. You won’t develop toxicity from eating magnesium-rich foods like spinach, nuts, or beans. The concern is concentrated doses from pills, powders, or liquids.
What Happens as Levels Keep Rising
Your body normally keeps blood magnesium between about 1.7 and 2.3 mg/dL. Anything above 2.6 mg/dL is considered elevated. What happens next depends on how far above normal your levels climb.
Mild elevation (below 7 mg/dL): Many people have no obvious symptoms at this stage. Low blood pressure that doesn’t respond well to treatment may be one of the earliest signals. You might also feel nauseated, dizzy, or slightly weak.
Moderate elevation (7 to 12 mg/dL): Symptoms become harder to ignore. Confusion, significant muscle weakness, difficulty breathing, and drowsiness are typical. Your heart’s electrical activity starts to change at this range, with the signals that coordinate each heartbeat slowing and widening. Your reflexes fade. By the time levels approach 12 mg/dL, your normal knee-jerk reflex disappears entirely, which is a red flag that doctors specifically test for.
Severe elevation (above 12 mg/dL): This is a medical emergency. Muscle paralysis, including the muscles that control breathing, can set in. Extreme drowsiness progressing to coma is possible. Cardiac arrest, where the heart stops beating, can occur when blood magnesium exceeds roughly 15 mg/dL.
How Excess Magnesium Disrupts Your Body
Magnesium and calcium compete for the same spots in your nerve and muscle cells. Under normal conditions, calcium triggers muscle contraction and nerve signaling. When magnesium levels get too high, the excess magnesium blocks calcium from doing its job. Your nerves can’t fire properly, your muscles can’t contract, and the electrical signals in your heart slow down. This is why the symptoms progress from weak reflexes to paralysis to cardiac arrest: it’s a continuum of the same basic problem, with magnesium increasingly silencing your neuromuscular system.
This mechanism is also why calcium is the first-line emergency treatment for severe magnesium toxicity. Intravenous calcium directly counteracts the magnesium blocking those receptor sites, temporarily restoring nerve and muscle function while the body works to clear the excess.
Who Is Most at Risk
Healthy kidneys are remarkably efficient at filtering out extra magnesium. If your kidney function is normal, it’s difficult (though not impossible) to reach dangerous blood levels from oral supplements alone, because your kidneys excrete the excess and your gut stops absorbing it well before toxicity sets in. You’ll almost certainly get diarrhea long before you’re in danger.
The risk changes dramatically for people with kidney disease. When kidney filtration drops below about 30% of normal capacity, the body loses its ability to clear magnesium efficiently. Levels can build up quickly, even from standard supplement doses or magnesium-containing antacids and laxatives. People with chronic kidney disease should be especially careful with any magnesium-containing product.
Older adults are also at higher risk, partly because kidney function naturally declines with age, sometimes without obvious symptoms. Anyone already taking medications that contain magnesium (certain antacids, laxatives, or heartburn remedies) can inadvertently stack doses beyond the safe limit without realizing they’re consuming supplemental magnesium from multiple sources.
What Mild Overdose Looks Like in Practice
For most people reading this article, the realistic scenario isn’t cardiac arrest. It’s taking a magnesium supplement at a dose above 350 mg, or combining a supplement with a magnesium-containing laxative, and ending up with uncomfortable diarrhea, stomach cramps, and nausea. These symptoms usually resolve within a day or two of stopping or reducing the dose.
If you’re experiencing persistent diarrhea from a magnesium supplement, try switching to a better-absorbed form like magnesium glycinate, which is less likely to cause gut issues than citrate or oxide. Splitting your dose across the day rather than taking it all at once also helps your body absorb more and leave less sitting in your intestines.
Symptoms that go beyond digestive discomfort, particularly noticeable drops in blood pressure, unusual muscle weakness, or difficulty breathing, signal that magnesium levels may have climbed into a concerning range. This is more likely if you have reduced kidney function or have taken very large amounts.