Short answer: The NIH Office of Dietary Supplements says forms of magnesium that dissolve well in liquid, including magnesium citrate, tend to be absorbed more completely than less soluble forms such as magnesium oxide. The evidence it cites comes from small studies.* Absorption is only one part of choosing a supplement, so this guide also covers daily amounts, the upper limit for supplements, and when to check with a professional.
What is a "form" of magnesium?
Magnesium in a supplement is always paired with another substance. Magnesium citrate is magnesium paired with citric acid. Magnesium oxide is magnesium paired with oxygen. These pairings are called forms, or salts, and they dissolve in liquid to different degrees. The NIH links that difference to how completely a form is absorbed in the gut.*
What the NIH says about absorption
In its fact sheet for health professionals, the NIH Office of Dietary Supplements (ODS) explains that magnesium forms that dissolve well in liquid are more completely absorbed in the gut than less soluble forms. It adds that the aspartate, citrate, lactate, and chloride forms tend to have higher bioavailability than magnesium oxide and magnesium sulfate, and it cites small studies.* The ODS consumer fact sheet also lists citrate among the forms that are more easily absorbed by the body.*
| Form | Paired with | What the NIH ODS says |
|---|---|---|
| Magnesium citrate | Citric acid | Listed among forms that tend to have higher bioavailability than oxide, based on small studies* |
| Magnesium oxide | Oxygen | Listed, with sulfate, as having lower bioavailability than citrate, based on small studies* |
What this does not tell you
The studies the NIH cites are small, and the fact sheet does not give an absorption percentage for any specific supplement. It also does not discuss other forms people ask about, such as magnesium glycinate, so this guide does not compare them. Absorption is one factor among several. The amount in each serving and how you take it matter too, and a pharmacist or doctor can help you weigh them.
How much magnesium do adults need?
The NIH ODS lists recommended daily amounts for adults from all sources, including food, drinks, supplements, and medicines: 400 to 420 mg for men and 310 to 320 mg for women. Separately, the upper limit for magnesium from supplements and medicines is 350 mg per day for adults. That limit does not include magnesium found naturally in food and beverages, and the NIH advises not to go above it unless a health care provider recommends it.
Safety: know the limit and talk to a professional
According to the NIH, high doses of magnesium from supplements and medicines can cause diarrhea, nausea, and abdominal cramping. Extremely high intakes can be dangerous, and the NIH notes the risk is greater for people with impaired kidney function. Magnesium supplements can also interact with some medicines, such as certain antibiotics and some prescription bone medicines, which may be absorbed less well when taken too close in time to a magnesium product. Very high doses of zinc supplements can interfere with magnesium absorption as well.
The FDA advises consumers to talk to their doctor, pharmacist, or other health care professional before deciding to use a dietary supplement. If you take prescription medicines, ask your pharmacist how to space them from any magnesium supplement.
Where PhiNaturals Magnesium Citrate fits
PhiNaturals Magnesium Citrate is a vegan capsule made with 400 mg magnesium citrate, cellulose, and a vegetable capsule (HPMC). Whichever supplement you choose, follow the directions on its label and stay within the serving size.
Sources
- NIH Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. Updated January 6, 2026.
- NIH Office of Dietary Supplements. Magnesium: Fact Sheet for Consumers. Updated March 22, 2021.
- U.S. Food and Drug Administration. Information for Consumers on Using Dietary Supplements. Content current as of October 21, 2022.
Written by the PhiNaturals team. Sources reviewed October 30, 2026. This article is for general education and is not medical advice.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.