Is Magnesium Oxide 400mg the Same as Magnesium? A Crucial Difference

November 15, 2025 •

3 min read

According to the National Institutes of Health, over half of the U.S. population has inadequate magnesium intake from diet alone. This raises a common question for many who turn to supplements: is magnesium oxide 400mg the same as magnesium, or is there a crucial difference to be aware of? Understanding the distinction between the compound and the elemental mineral is key to knowing how your body uses the supplement you take.

Quick Summary

A 400mg magnesium oxide supplement contains a certain amount of elemental magnesium, but the total pill weight is not the same as the bioavailable mineral content. Its low solubility makes it more effective as a laxative than for correcting systemic deficiencies, unlike other magnesium forms.

Key Points

Magnesium Oxide is a compound, not pure magnesium: A 400mg tablet refers to the weight of the compound, which includes both magnesium and oxygen, not the elemental magnesium alone.
Low bioavailability affects absorption: Magnesium oxide has a low absorption rate compared to other forms, making it less effective for correcting systemic deficiencies.
Elemental magnesium is the key metric: Always check the 'Supplement Facts' label for the amount of elemental magnesium, which indicates the usable amount of the mineral in the pill.
Primary use is as a laxative: Due to its poor absorption, magnesium oxide exerts an osmotic effect in the intestines and is commonly used for relieving constipation.
Other forms are better for systemic health: For general supplementation and targeting specific health benefits, highly bioavailable forms like magnesium citrate or glycinate are often better choices.
Consult a professional before use: Anyone with kidney issues or who is taking other medications should consult a healthcare provider due to the risk of hypermagnesemia.

The Chemical Reality: Why Magnesium Oxide is Not Pure Magnesium

At its core, the difference between "magnesium" and "magnesium oxide" comes down to chemistry. Magnesium is an element (Mg), a naturally occurring mineral essential for numerous bodily functions. Magnesium oxide (MgO), on the other hand, is a chemical compound created when the element magnesium is combined with oxygen. This compound is used to create supplements, but the 400mg dosage refers to the total weight of the magnesium-oxygen compound, not the amount of pure, usable magnesium your body will absorb.

Understanding Elemental Magnesium

Every magnesium supplement should list its 'elemental magnesium' content on the nutrition facts label. Elemental magnesium is the total amount of pure magnesium contained within the compound. For a 400mg tablet of magnesium oxide, the actual elemental magnesium is approximately 240mg. This is a critical point that many people overlook when purchasing supplements.

The Bioavailability Issue: What Your Body Actually Absorbs

Beyond the elemental content, the next major factor is bioavailability—the rate and extent to which your body can absorb and utilize the magnesium. This is where magnesium oxide has a significant drawback compared to other forms. Magnesium oxide has a notably low absorption rate, with some research indicating that only a small fraction is absorbed by the intestines. This low solubility is precisely what gives magnesium oxide its other common use: as a laxative. Its osmotic effect draws water into the intestines, softening stool and promoting bowel movements. For someone with a magnesium deficiency, this means a 400mg magnesium oxide tablet may primarily have a laxative effect rather than effectively replenishing magnesium stores throughout the body. Other forms of magnesium, such as magnesium citrate, malate, or glycinate, have much higher bioavailability and are therefore better suited for correcting deficiencies.

Other Magnesium Forms to Consider

If your goal is to boost your magnesium levels for systemic health benefits, such as supporting nerve and muscle function, bone health, or improving sleep, exploring other forms is highly recommended. For instance, magnesium citrate is highly soluble and known for good absorption. Magnesium glycinate is another popular choice, particularly for those with sensitive stomachs, as it is easily absorbed and less likely to cause digestive upset. A healthcare professional can help determine the best form and dosage for your specific needs.

How to Choose the Right Supplement

When selecting a magnesium supplement, don't just look at the total milligrams on the front of the package. Turn it over and check the 'Supplement Facts' panel for the amount of 'Elemental Magnesium.' Consider your primary goal for supplementation. Is it for constipation relief, or to address a deficiency? Your answer will guide you to the most appropriate form of magnesium.

Comparison Table: Magnesium Oxide vs. Other Forms


Feature	Magnesium Oxide	Magnesium Citrate	Magnesium Glycinate
Elemental Content	High (Approx. 60%)	Moderate (Approx. 11%)	Moderate
Bioavailability	Low	High	High
Primary Use	Laxative, Antacid, Inexpensive Supplement	Addressing Deficiency, Constipation	Relaxation, Sleep Support
Digestive Impact	Can cause diarrhea	Can cause laxative effect	Gentle on the stomach
Cost	Generally affordable	Moderate	Often more expensive

Potential Side Effects and Precautions

While magnesium supplements are generally safe, high doses can lead to gastrointestinal side effects like diarrhea, nausea, and cramping, especially with magnesium oxide. For individuals with impaired kidney function, excessive intake of any form of magnesium can lead to hypermagnesemia, a serious condition where magnesium levels in the blood become dangerously high. It is crucial to consult with a healthcare professional before starting any new supplement regimen, particularly if you have pre-existing health conditions or are taking other medications. Some medications, like certain antibiotics, can have their absorption hindered by magnesium.

Conclusion: The Final Verdict

In summary, a 400mg dose of magnesium oxide is not the same as 400mg of elemental magnesium. The label refers to the full compound, which provides a lower amount of elemental magnesium and has a low bioavailability. While magnesium oxide can be a cost-effective supplement for some or an effective laxative, it is not the most efficient choice for correcting systemic magnesium deficiency due to its poor absorption. For those seeking to increase their body's magnesium levels for broader health benefits, more bioavailable forms like magnesium citrate or glycinate are generally preferable. Always read the nutrition label for elemental content and discuss your needs with a healthcare provider to ensure you choose the right supplement for your goals. For more in-depth nutritional information, you can consult a trusted source like the National Institutes of Health's Office of Dietary Supplements.

Frequently Asked Questions

Magnesium is a mineral element, while magnesium oxide is a chemical compound of magnesium and oxygen. A 400mg magnesium oxide pill contains less than 400mg of actual magnesium and is less absorbed by the body than other forms.

A 400mg dose of magnesium oxide typically contains about 240mg of elemental magnesium. This is the amount of pure magnesium available in the supplement before accounting for the body's absorption rate.

Magnesium oxide has a low solubility in water, which results in poor absorption in the intestines. This is why it is often used as a laxative rather than for correcting significant magnesium deficiencies.

For increasing systemic magnesium levels, supplements with higher bioavailability, such as magnesium citrate, glycinate, or malate, are more effective. Magnesium oxide is not the best choice due to its low absorption.

Magnesium oxide is commonly used as an over-the-counter antacid to relieve heartburn and acid indigestion, and as a laxative for relieving occasional constipation.

Yes, high doses of magnesium oxide, including 400mg, can cause gastrointestinal side effects such as diarrhea, nausea, and stomach cramping, especially in people with sensitive digestive systems.

Other forms like magnesium citrate and glycinate have higher bioavailability, meaning they are more readily absorbed and used by the body. They are also less likely to cause digestive upset compared to magnesium oxide, though they may be more expensive.