Magnesium supplements can cause side effects, most commonly affecting the digestive system. Diarrhea, nausea, and stomach pain are especially likely when magnesium is taken in doses above 350 milligrams per day. At much higher amounts, magnesium can lead to more serious reactions, including symptoms of toxicity such as low blood pressure and muscle weakness.

Common Side Effects of Magnesium

Gastrointestinal (GI) issues are the most frequently reported side effects associated with magnesium supplementation. These reactions are more likely to occur when intake exceeds 350 milligrams daily, which is the recommended upper limit for supplemental magnesium in adults.

Gastrointestinal Symptoms

The most common digestive side effects include diarrhea, nausea, vomiting, stomach pain, and abdominal cramping. Magnesium is often used as a treatment for constipation because it draws water into the intestines. This softens stool and makes bowel movements easier. However, this same mechanism can also lead to loose stools and digestive discomfort when too much magnesium is consumed.

Certain forms of magnesium are more likely to cause diarrhea and other GI symptoms. These include magnesium carbonate, magnesium chloride, magnesium oxide, and magnesium gluconate. The likelihood and severity of side effects increase as the dose rises.

Taking magnesium supplements with food may help reduce stomach irritation and lower the risk of nausea or cramping.

Magnesium Toxicity

Extremely high doses of magnesium—typically more than 5,000 milligrams per day—can result in magnesium toxicity. This occurs when blood magnesium levels rise above the normal range of 0.75–0.95 millimoles per liter and reach levels between 1.74 and 2.61 millimoles per liter.

Symptoms of magnesium toxicity may include:

• Low blood pressure (hypotension)

• Facial flushing or redness

• Confusion or mental changes

• Difficulty urinating

• Fatigue or extreme drowsiness

• Muscle weakness

• Trouble breathing

• Slow, fast, or irregular heartbeat

• Heart failure

• Coma

• Cardiac arrest

Magnesium toxicity is rare and most often occurs when magnesium is administered intravenously in a medical setting. However, very high doses of oral magnesium supplements can still cause toxicity, particularly in people with reduced kidney function.

Who Is at Higher Risk for Side Effects?

Certain individuals are more likely to experience side effects from magnesium supplements. This includes people with kidney disease, reduced kidney function, or a history of kidney failure. Those receiving dialysis and older adults are also at increased risk, as kidney function naturally declines with age.

Additionally, some forms of magnesium—such as magnesium oxide or magnesium sulfate—are more likely to cause digestive discomfort compared to other formulations.

What Is a Safe Magnesium Dose?

For adults, the upper safe limit for magnesium from supplements alone is 350 milligrams per day. This limit does not include magnesium obtained naturally from food sources such as leafy green vegetables, whole grains, nuts, seeds, and legumes.

The recommended daily intake of magnesium from both food and supplements varies by age, sex, and life stage:

• Men:

  • Ages 14–18: 410 mg

  • Ages 19–30: 400 mg

  • Ages 31 and older: 420 mg

• Women:

  • Ages 14–18: 360 mg

  • Ages 19–30: 310 mg

  • Ages 31 and older: 320 mg

• Pregnant individuals:

  • Ages 14–18: 400 mg

  • Ages 19–30: 350 mg

  • Ages 31 and older: 360 mg

• Breastfeeding individuals:

  • Ages 14–18: 360 mg

  • Ages 19–30: 310 mg

  • Ages 31 and older: 320 mg

When to See a Doctor

If you are taking magnesium supplements and develop symptoms such as severe weakness, breathing difficulty, confusion, or low blood pressure, stop taking the supplement and seek medical care immediately.

You should also contact a doctor right away if you notice blood in your stool or black, tarry stools after taking magnesium, as this may indicate gastrointestinal bleeding.

Before starting any supplement, it is always best to consult a healthcare provider to make sure it is safe, appropriate, and dosed correctly for your individual health needs.