Why does magnesium glycinate cause premature cardiac beats at a dose of 120 mg elemental magnesium?

Magnesium Glycinate and Premature Cardiac Beats: Understanding the Paradox

Magnesium glycinate at 120 mg elemental magnesium should not cause skipped heartbeats—in fact, magnesium is used therapeutically to suppress premature cardiac beats and ventricular arrhythmias. 1 The symptom you're experiencing is likely unrelated to the magnesium supplementation itself, or may represent a paradoxical response requiring investigation of underlying cardiac conditions or concurrent electrolyte imbalances.

Why This Doesn't Make Physiological Sense

Magnesium's established cardiac effects are antiarrhythmic, not proarrhythmic. The evidence consistently demonstrates that:

• Oral magnesium supplementation (260 mg elemental magnesium daily) significantly reduces premature ventricular and supraventricular complexes by >70% in 76.6% of patients, with symptom improvement in 93.3% of treated patients 1
• Magnesium increases the ventricular threshold for fibrillation and stabilizes cardiac membranes by regulating calcium and potassium channel function 2, 3
• Hypomagnesemia—not magnesium supplementation—is what causes cardiac arrhythmias, including premature beats, by destabilizing myocyte membranes 3, 4

What May Actually Be Happening

Underlying Electrolyte Imbalances

• Check your potassium and calcium levels immediately. Magnesium deficiency commonly coexists with hypokalemia and hypocalcemia, and correcting magnesium can unmask or transiently worsen these other deficiencies before they normalize 3, 4
• Hypomagnesemia renders concurrent hypokalemia resistant to correction, and normalizing magnesium is essential for successful potassium repletion 3
• If you have borderline low potassium (even if technically "normal"), correcting magnesium may temporarily increase renal potassium excretion before homeostasis is restored 4

Pre-existing Cardiac Conditions

• Obtain an ECG to evaluate for QTc prolongation, structural heart disease, or underlying conduction abnormalities. 5 The premature beats may have been present before supplementation but only became noticeable after you started paying attention to your heart rhythm
• Consider screening for genetic long QT syndrome variants if you have a family history of sudden cardiac death or syncope 6

Concurrent Medications or Substances

• Review all medications, particularly diuretics, proton pump inhibitors, digoxin, or QT-prolonging drugs. 6, 5 These can cause or worsen electrolyte disturbances and arrhythmias
• Loop and thiazide diuretics cause substantial magnesium and potassium loss, which may be contributing to your symptoms 4, 5
• Alcohol use disorder is a major cause of magnesium deficiency and cardiac arrhythmias 3

The Dose Is Not the Problem

120 mg of elemental magnesium is a modest, safe dose—far below levels that cause any cardiac toxicity. 6

• Therapeutic doses for treating arrhythmias range from 260 mg daily orally 1 to 1-2 grams intravenously for life-threatening ventricular arrhythmias 6, 3
• Magnesium toxicity (which causes bradycardia and heart block, not premature beats) only occurs at serum levels of 6-10 mmol/L, typically requiring massive intravenous doses or severe renal failure 6, 5
• The risk of toxicity from oral supplementation at 120 mg daily is essentially zero in patients with normal kidney function 6

What You Should Do Now

Immediate Actions

1. Continue the magnesium supplementation—stopping it may worsen any underlying deficiency 5, 3
2. Schedule an ECG and comprehensive metabolic panel including magnesium, potassium, calcium, and creatinine 5, 3
3. Document the premature beats with a 24-hour Holter monitor to quantify their frequency and determine if they're truly new 1

Electrolyte Optimization

• Target serum magnesium ≥2.0 mEq/L (0.82 mmol/L) for patients with ventricular arrhythmias 3
• Maintain serum potassium between 4.5-5.0 mEq/L, as lower levels increase arrhythmia risk even when technically "normal" 6
• Correct any concurrent hypocalcemia, but only after magnesium is normalized 5, 3

Common Pitfalls to Avoid

• Don't assume normal serum magnesium excludes deficiency—serum levels don't reflect intracellular stores, and 60-65% of patients with heart failure have intracellular magnesium depletion despite normal serum levels 4, 5
• Don't supplement calcium and magnesium together—they inhibit each other's absorption; separate by at least 2 hours 5
• Don't stop the magnesium based on temporal association alone—correlation does not equal causation, and magnesium deficiency itself causes the exact symptoms you're experiencing 3, 1

The Bottom Line

The premature beats are almost certainly not caused by the magnesium glycinate. The evidence overwhelmingly supports that magnesium supplementation suppresses—rather than causes—premature cardiac beats. 1, 2 You need a thorough cardiac and electrolyte evaluation to identify the true cause, which may include underlying magnesium deficiency (requiring higher doses, not discontinuation), concurrent potassium or calcium abnormalities, pre-existing cardiac conditions, or medication effects. 5, 3, 4