Short answer: The NIH sets the upper limit for magnesium from dietary supplements and medicines at 350 mg a day for everyone age 9 and older, and lower for young children. High amounts from supplements can cause diarrhea, nausea, and abdominal cramping. Magnesium can also change how some medicines are absorbed, so timing matters and a pharmacist is a good person to ask.
The upper limit for supplemental magnesium
The NIH Office of Dietary Supplements publishes a Tolerable Upper Intake Level (UL) for magnesium. A UL is the highest daily amount from supplements and medicines that is unlikely to cause problems for most healthy people. It is a ceiling, not a target.
| Age group | Upper limit from supplements and medicines |
|---|---|
| Birth to 12 months | None established |
| 1 to 3 years | 65 mg |
| 4 to 8 years | 110 mg |
| 9 to 18 years | 350 mg |
| Adults (19 and older) | 350 mg |
| Pregnancy and lactation (14 to 50 years) | 350 mg |
These limits count only magnesium from dietary supplements and medicines. Magnesium that occurs naturally in food and drinks is not included, and the NIH describes the risk of getting too much from food as low among healthy people. For what everyday foods provide, see our guide to magnesium-rich foods, and for daily amounts by age, see how much magnesium you need per day.
What high amounts from supplements can cause
According to the NIH, high doses of magnesium from supplements or medicines can cause diarrhea, nausea, and abdominal cramping. The NIH names magnesium carbonate, chloride, gluconate, and oxide as the forms most often linked to diarrhea. Citrate is not on that list, but that does not make any form free of these effects at large amounts. Extremely high intakes can cause more serious effects, which is one reason upper limits exist.
The NIH also notes that the risk of too much magnesium is greater for people with impaired kidney function. If you have any medical condition, ask your doctor or pharmacist before you take a magnesium supplement.
Medicines that can interact with magnesium
The NIH lists a few types of medicine that can interact with magnesium. This table summarizes what its fact sheets say. It is not a complete list of medicines or a substitute for advice from your pharmacist.
| Medicine type | What the NIH says |
|---|---|
| Oral bisphosphonates (bone medicines, such as alendronate) | They may not be absorbed well when taken too close to a magnesium supplement. Spacing doses at least 2 hours before or after can minimize this. |
| Tetracycline and quinolone antibiotics (such as doxycycline, ciprofloxacin, levofloxacin) | Magnesium can reduce their absorption. The NIH advises taking these antibiotics at least 2 hours before or 4 to 6 hours after a magnesium-containing supplement. |
| Diuretics (water pills) | Loop and thiazide diuretics can cause the body to lose magnesium. Potassium-sparing diuretics reduce magnesium excretion. |
| Proton pump inhibitors (stomach acid medicines) | Taken for long periods, typically more than a year, they can lower magnesium levels in the blood. |
The NIH consumer fact sheet also says very high doses of zinc supplements can interfere with the body's ability to absorb and regulate magnesium.
The NIH advises people who take any of these medicines regularly to discuss their magnesium intake with their health care providers. The FDA gives similar advice for supplements in general: be informed, and talk to your doctor, pharmacist, or other health care professional before you decide to buy or use one.
Comparing a product to these numbers
The upper limits above are expressed as magnesium. Supplement labels are not always written the same way: some list the weight of the whole compound (such as magnesium citrate), while others list the amount of magnesium it supplies. Check the Supplement Facts panel to see which one the label uses, and ask a pharmacist if you are not sure how a label compares to the NIH numbers.
A short checklist before you start
- Read the Supplement Facts panel and follow the serving size and directions on the label.
- Tell your doctor and pharmacist about every supplement and medicine you take, including over-the-counter ones.
- If you take any of the medicine types above, ask how to space your doses.
- Keep supplements out of the reach of children. The upper limit for young children is much lower than for adults.
- If you notice diarrhea, nausea, or cramping after you start a supplement, talk with a pharmacist or doctor about the amount you are taking.
Where PhiNaturals Magnesium Citrate fits
PhiNaturals Magnesium Citrate capsules are made with 400 mg magnesium citrate, cellulose, and a vegetable capsule shell (HPMC), and are packaged in the USA in a GMP facility. Follow the directions on the bottle, and check with a doctor or pharmacist first if you are pregnant, nursing, or taking medicines. See the product page.
Sources
- NIH Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals (updated January 6, 2026). See "Health Risks from Excessive Magnesium," "Interactions with Medications," and Table 3.
- 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.