Magnesium: The Missing Link to

Magnesium oxide 400 Mg Side Effects

Not all side effects for magnesium oxide may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.

For the Consumer

Applies to magnesium oxide: capsules, tablets

Check with your doctor if any of these most COMMON side effects persist or become bothersome:


Seek medical attention right away if any of these SEVERE side effects occur while taking magnesium oxide:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); black, tarry stools; nausea; slow reflexes; vomit that looks like coffee grounds.

For Healthcare Professionals

Applies to magnesium oxide: compounding powder, oral capsule, oral tablet


Other side effects have included effects due to toxicity (hypermagnesemia). Mild hypermagnesemia is generally well-tolerated. Moderate or severe hypermagnesemia primarily affects the nervous and cardiovascular systems. The effects of hypermagnesemia may be worsened by the presence of hypocalcemia, especially in patients with uremia.


Gastrointestinal side effects have included diarrhea. Magnesium toxicity has been associated with nausea (magnesium levels of 2.0 to 2.5 mg/dL or 4 to 5 mEq/L) and rare cases of paralytic ileus (magnesium levels greater than 5 mEq/L).

Nervous system

Nervous system side effects have resulted from the suppression of neuromuscular transmission in the CNS and at the neuromuscular junction by magnesium (antagonized by calcium). The degree of severity of these side effects has been dependent on the serum magnesium level. Clinically, if serum magnesium levels increase to 4 to 7 mEq/L, there may be a decrease in tendon reflexes, muscle weakness and/or mental confusion/sedation. At levels of 7 to 10 mEq/L the respiratory rate slows and the blood pressure falls. At levels of 10 to 15 mEq/L there is usually profound mental depression, areflexia, coma and respiratory paralysis. Magnesium also has a curare-like effect at the neuromuscular junction at serum levels above 10 mEq/L. Death is not uncommon when serum magnesium levels rise to 15 mEq/L.


Cardiovascular side effects have included hypotension, depressed myocardial conductivity, and bradyarrhythmias.

The cardiovascular consequences of hypermagnesemia are due to peripheral vasodilation. Hypotension may be observed when serum Mg levels rise to 7 to 10 mEq/L, becoming severe when serum Mg levels rise above 10 mEq/L. Magnesium can also depress myocardial conductivity and cause bradyarrhythmias at levels greater than 10 mEq/L. While some patients are inexplicably able to tolerate extraordinary magnesium levels, there is a significant risk of asystole when levels rise to 25 mEq/L. The risk of cardiotoxicity from hypermagnesemia is increased in the presence of hypocalcemia, hyperkalemia, acidosis, digitalis therapy, and renal insufficiency.


Acute hypermagnesemia may cause hypocalcemia due to suppression of the release of parathyroid hormone (PTH) and competition for renal tubular reabsorption between calcium (Ca) and magnesium. The latter can lead to decreased Ca reabsorption and hypercalciuria, which aggravates the hypocalcemia produced by decreased release of PTH.

Metabolic side effects have included hypocalcemia.

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