Magnesium is a vital mineral your body needs to regulate muscle and nerve function, support bone strength, and maintain healthy blood pressure. While magnesium supplements are generally safe for most people, taking too much—or the wrong type—can lead to uncomfortable or even serious side effects.

This guide covers common side effects, risk factors, and when to seek medical advice if you're supplementing with magnesium.

Common Side Effects of Magnesium Supplements

Gastrointestinal (GI) Issues

The most frequently reported side effects from magnesium supplementation are related to the digestive system. These may include:

  • Diarrhea

  • Nausea

  • Vomiting

  • Stomach cramps

  • Abdominal discomfort

These effects are more likely to occur with doses above 350 mg/day—the upper limit for magnesium supplements in adults. Some forms of magnesium, such as magnesium oxide, gluconate, chloride, and carbonate, are more likely to cause diarrhea due to their laxative effects.

If you experience GI upset, try taking magnesium with food, or switch to a gentler form like magnesium glycinate.

Magnesium Toxicity: What Happens When You Take Too Much

Toxic levels of magnesium are rare but can occur, especially if you consume extremely high doses (more than 5,000 mg per day) or if your kidneys can't eliminate excess magnesium efficiently.

Symptoms of Magnesium Overload May Include:

  • Low blood pressure (hypotension)

  • Muscle weakness

  • Fatigue or confusion

  • Flushing and facial redness

  • Irregular heartbeat

  • Breathing difficulties

  • Decreased urine output

  • Coma or cardiac arrest (in extreme cases)

Toxicity is more commonly associated with intravenous magnesium or excessive supplementation in people with impaired kidney function.

Who’s at Higher Risk of Side Effects?

Certain individuals are more likely to experience magnesium-related side effects, including:

  • People with kidney disease or kidney failure

  • Older adults, as kidney function naturally declines with age

  • Individuals on dialysis

  • People taking magnesium sulfate or oxide, which are harsher on the digestive tract

If you fall into any of these categories, consult your doctor before starting magnesium supplements.

How Much Magnesium Is Too Much?

Supplemental Limit:

  • The tolerable upper intake level (UL) for magnesium from supplements is 350 mg per day for adults. This does not include magnesium from food.

Daily Recommended Intake (Diet + Supplements):

Men:

  • 14–18 years: 410 mg

  • 19–30 years: 400 mg

  • 31+ years: 420 mg

Women:

  • 14–18 years: 360 mg

  • 19–30 years: 310 mg

  • 31+ years: 320 mg

Pregnant Women:

  • 14–18 years: 400 mg

  • 19–30 years: 350 mg

  • 31+ years: 360 mg

Breastfeeding Women:

  • 14–18 years: 360 mg

  • 19–30 years: 310 mg

  • 31+ years: 320 mg

Be sure to read product labels carefully and consider how much magnesium you’re already getting from your diet.

When to Seek Medical Help

You should stop taking magnesium supplements and contact a healthcare provider if you experience:

  • Persistent diarrhea, cramping, or vomiting

  • Symptoms of low blood pressure (e.g., dizziness, fatigue)

  • Blood in your stool or black, tar-like stools

  • Muscle weakness or slowed reflexes

  • Unusual heartbeat or chest discomfort

These may be signs of magnesium toxicity or another serious underlying issue.

Final Thoughts

Magnesium is essential for many bodily processes, but too much can cause gastrointestinal discomfort or, in rare cases, toxicity. The majority of side effects occur when taking high doses—particularly above the 350 mg/day limit for supplements.

Before starting magnesium—especially if you have a medical condition, take medications, or are unsure about dosage—talk with your healthcare provider to ensure it's safe and appropriate for your needs.