What's 500 mg of magnesium too much?

January 1, 2026 •

4 min read

According to the NIH Office of Dietary Supplements, the Tolerable Upper Intake Level (UL) for supplemental magnesium in adults is 350 mg per day. A 500 mg dose therefore exceeds this official threshold, increasing the risk of adverse side effects, particularly for those with underlying health conditions.

Quick Summary

This article explores whether a 500 mg magnesium supplement is excessive for healthy adults, detailing the official tolerable upper intake level, discussing common side effects like diarrhea, and explaining the risks of magnesium toxicity.

Key Points

Tolerable Upper Limit: The official Tolerable Upper Intake Level for magnesium from supplements is 350 mg for adults, making a 500 mg dose potentially excessive.
Gastrointestinal Side Effects: A 500 mg dose of magnesium is likely to cause mild to moderate digestive side effects such as diarrhea, nausea, and cramping, especially with less bioavailable forms.
Risk for Kidney Impairment: Individuals with kidney disease are at a much higher risk for magnesium toxicity (hypermagnesemia) because their kidneys cannot effectively remove excess magnesium.
Form Matters: The type of magnesium supplement influences side effects; magnesium oxide is less absorbed and more likely to cause diarrhea, while magnesium glycinate is better tolerated.
Consult a Doctor: It is always best to consult a healthcare professional before beginning a high-dose supplement regimen, especially when exceeding the 350 mg UL.
Toxicity is Serious: While rare, severe magnesium toxicity can lead to serious symptoms like low blood pressure, muscle weakness, and irregular heartbeat.

The Official Tolerable Upper Intake Level (UL)

For healthy adults, the Tolerable Upper Intake Level (UL) for magnesium from supplements is set at 350 mg per day. The UL is the maximum daily intake that is unlikely to cause adverse health effects. While a 500 mg dose is higher than this guideline, it is important to distinguish between magnesium from supplements and magnesium obtained from food. The kidneys in a healthy person are highly effective at flushing out excess magnesium from dietary sources, but high-dose supplements can overwhelm this system. A dose of 500 mg or more is often used under medical supervision to treat specific conditions, such as migraines.

How Your Health Affects Magnesium Tolerance

For most healthy individuals, a 500 mg dose may simply cause mild gastrointestinal discomfort, with the body expelling the excess through the laxative effect of unabsorbed magnesium. However, certain pre-existing conditions significantly increase the risk of more serious issues, known as hypermagnesemia or magnesium toxicity. Individuals with kidney disease are especially vulnerable, as their impaired renal function prevents the efficient removal of excess magnesium from the body. It is crucial to consult a healthcare provider before starting any high-dose magnesium regimen, especially if you have chronic health issues.

Factors Influencing Your Magnesium Needs

Kidney Function: Those with impaired kidney function must be extremely cautious with supplemental magnesium as their body cannot filter it properly, leading to potential toxicity.
Underlying Health Conditions: Conditions such as heart block, myasthenia gravis, or specific gastrointestinal disorders can make high-dose magnesium more problematic and require medical supervision.
Form of Magnesium: Different forms of magnesium have varying bioavailability. Less absorbable forms, like magnesium oxide, can cause more prominent gastrointestinal side effects at lower doses because more of the unabsorbed mineral remains in the gut. More bioavailable forms, like magnesium glycinate, may be better tolerated.

Common Side Effects of High-Dose Magnesium

Even for healthy individuals, taking 500 mg of magnesium can lead to noticeable side effects, with gastrointestinal symptoms being the most common. These side effects are often the body's way of signaling an intake level higher than needed.

Gastrointestinal Upset

High doses of magnesium commonly result in a laxative effect, which can cause:

Diarrhea
Nausea
Abdominal cramping
Stomach pain

Other Mild Symptoms

Beyond digestive issues, some people may experience other mild symptoms, including:

Lethargy
Confusion
Facial flushing

Magnesium Forms: A Comparison

Choosing the right form of magnesium is critical for managing absorption and potential side effects. Bioavailability, or the amount of magnesium the body can absorb and use, varies greatly by form. Below is a comparison of common magnesium types.


Magnesium Form	Bioavailability	Common Uses	Potential Side Effects	Notes
Magnesium Oxide	Low	Constipation relief, antacid	High risk of diarrhea, stomach upset	Least absorbable; high elemental magnesium content creates a strong laxative effect.
Magnesium Citrate	Good	Constipation, general supplementation	Can cause diarrhea, especially at higher doses	Moderately absorbable, with a noticeable laxative effect.
Magnesium Glycinate	High	Relaxation, improved sleep	Less likely to cause GI issues than citrate or oxide	Better tolerated by the gut, making it ideal for those sensitive to stomach upset.
Magnesium L-Threonate	High	Memory, brain health	Generally well-tolerated, specific effects depend on dosage	Specialized form designed to cross the blood-brain barrier.

Severe Magnesium Toxicity (Hypermagnesemia)

While rare in healthy individuals, hypermagnesemia is a serious condition that can occur from excessively high doses, particularly in people with impaired kidney function. Symptoms of severe magnesium toxicity include:

Profound muscle weakness or paralysis
Low blood pressure (hypotension)
Irregular heartbeat (arrhythmia)
Difficulty breathing
Lethargy and confusion leading to coma
Cardiac arrest in extreme cases

Foods Rich in Magnesium

Before resorting to high-dose supplements, most people can meet their daily magnesium needs through a balanced diet. Many nutritious foods are excellent sources of this essential mineral:

Leafy Greens: Spinach, Swiss chard, and kale
Nuts and Seeds: Almonds, cashews, peanuts, and pumpkin seeds
Legumes: Black beans, lima beans, and soybeans
Whole Grains: Brown rice, whole grain bread, and quinoa
Other Sources: Dark chocolate (at least 70% cacao), avocados, bananas, and salmon

Conclusion

For most healthy adults, taking a 500 mg magnesium supplement is more than the recommended Tolerable Upper Intake Level of 350 mg and may cause unpleasant gastrointestinal side effects like diarrhea. The risk of serious magnesium toxicity (hypermagnesemia) is low for those with normal kidney function, but it is a serious concern for individuals with renal impairment or other underlying health issues. Given the varying bioavailability of different magnesium forms and individual health differences, it is highly recommended to consult a healthcare professional before exceeding the 350 mg supplemental limit to determine the appropriate dosage and form for your specific needs.

Learn more about magnesium from the authoritative health resources at the National Institutes of Health.

Frequently Asked Questions

For a healthy person, taking 500 mg of magnesium is unlikely to be severely harmful, but it is over the 350 mg Tolerable Upper Intake Level (UL) for supplements. This dose can cause gastrointestinal side effects like diarrhea and nausea.

The most common side effects include diarrhea, abdominal cramping, nausea, and vomiting. Less common effects may involve lethargy or facial flushing.

Yes, the form of magnesium matters. Less bioavailable forms like magnesium oxide are more likely to cause diarrhea because more unabsorbed mineral remains in the gut. Higher bioavailability forms like magnesium glycinate may cause fewer gastrointestinal issues.

The Tolerable Upper Intake Level (UL) for supplemental magnesium in adults is 350 mg per day. This limit does not apply to magnesium from food sources.

Symptoms of severe toxicity (hypermagnesemia) are rare but include low blood pressure, muscle weakness, confusion, irregular heartbeat, difficulty breathing, and in very serious cases, cardiac arrest.

Individuals with impaired kidney function are at the highest risk for magnesium toxicity because their bodies cannot efficiently excrete excess magnesium. It is also a concern for those taking high doses for medical reasons.

No, it is highly unlikely to consume too much magnesium from food alone. A healthy body can regulate and excrete any excess magnesium ingested through diet.