Can you take too much magnesium?

The upper limit for supplemental magnesium

The Food and Nutrition Board sets a Tolerable Upper Intake Level (UL), the most you can take daily without likely harm. For magnesium, the UL covers only supplements and medicines. Healthy kidneys clear extra magnesium from food in the urine, so food has no limit.

The UL can look lower than the daily recommendation (310 to 420 mg for adults). That's because the recommendation counts food, while the UL counts only pills, powders and medicines. Add up every source: multivitamins, magnesium supplements, antacids and laxatives.

Signs you've taken too much

The first sign is usually diarrhea, sometimes with nausea and stomach cramps. Magnesium carbonate, chloride, gluconate and oxide are the forms most often reported to cause it. Unabsorbed magnesium pulls water into the bowel, which is also why some forms are sold as laxatives.

True magnesium toxicity is much less common. Signs can include low blood pressure, nausea, vomiting, facial flushing, trouble passing urine, a slowed gut, low mood and unusual tiredness. It can progress to muscle weakness, trouble breathing, an irregular heartbeat and cardiac arrest. A few deaths have been reported.

Call 911 if someone who has taken magnesium collapses, has a seizure, has trouble breathing, can't be woken up, or is very weak with a slow or irregular heartbeat. For a possible overdose without those signs, call Poison Control at 1-800-222-1222.

Why kidney problems raise the risk

Your kidneys are how your body gets rid of extra magnesium. When kidney function is reduced, or the kidneys fail, magnesium can build up even from amounts that are safe for others. If you have kidney disease or a low eGFR, ask your clinician before taking magnesium supplements, laxatives or antacids.

Some medicines matter too. Potassium-sparing diuretics, such as amiloride and spironolactone, reduce how much magnesium you lose in urine. Other drugs, like loop and thiazide diuretics and long-term acid reducers (PPIs), can lower magnesium instead. If you take any of these, ask your clinician whether your level should be checked.

Hidden magnesium in laxatives and antacids

Milk of magnesia (magnesium hydroxide) and magnesium citrate are laxatives, and some antacids contain magnesium. The toxicity cases on record mostly involved very large doses of these products.

• Don't use magnesium citrate or magnesium hydroxide as a laxative for more than 1 week without a clinician's advice.
• Tell your clinician first if you have kidney disease or are on a magnesium-restricted diet.
• With magnesium citrate, take other medicines at least 2 hours before or after it.
• Stop and call a clinician if you see blood in your stool or can't have a bowel movement after using one.

Need a daily supplement rather than a laxative? See magnesium glycinate benefits and magnesium glycinate vs citrate.

Spacing magnesium from other medicines

• Oral bisphosphonates for osteoporosis, such as alendronate: take magnesium at least 2 hours before or after.
• Tetracycline and quinolone antibiotics, such as doxycycline or ciprofloxacin: take the antibiotic at least 2 hours before or 4 to 6 hours after magnesium.

Never stop or change a prescribed medicine to make room for a supplement. Ask your prescriber or pharmacist about timing.