What are the potential side effects of magnesium supplementation?

Magnesium Side Effects

Magnesium supplementation commonly causes gastrointestinal side effects including diarrhea, nausea, vomiting, and abdominal distension, while excessive doses can lead to life-threatening toxicity manifesting as cardiac arrhythmias, respiratory depression, and hypotension. 1, 2

Common Side Effects (Therapeutic Doses)

Gastrointestinal Effects

• Diarrhea is the most frequent side effect of oral magnesium supplementation, occurring because most magnesium salts are poorly absorbed and draw water into the intestinal lumen 2, 3
• Abdominal distension and cramping can occur with standard supplementation doses 2
• Nausea and vomiting may develop, particularly with higher doses 1
• Paradoxically, excessive magnesium intake can cause severe diarrhea leading to hypomagnesemia through massive gastrointestinal losses 4

Tolerability Considerations

• Liquid or dissolvable magnesium products are generally better tolerated than pills 2
• Organic magnesium salts (aspartate, citrate, lactate) have better bioavailability and may cause fewer gastrointestinal symptoms than magnesium oxide or hydroxide 2
• Dividing doses throughout the day rather than single large doses improves tolerance 2

Serious Toxicity (Hypermagnesemia)

Cardiac Effects by Serum Level

• At magnesium levels of 2.5-5 mmol/L: ECG changes including prolonged PR interval, QRS widening, and QT prolongation 1
• At levels of 6-10 mmol/L: AV nodal conduction block, bradycardia, severe hypotension, and cardiac arrest 1

Neurological Effects

• Loss of deep tendon reflexes occurs at levels of 4-5 mmol/L 1
• Progressive sedation and altered mental status develop with rising levels 1
• Severe muscular weakness can progress to paralysis 1
• Respiratory depression is a critical complication requiring immediate intervention 1

Other Manifestations

• Skin flushing may occur 1
• Hypophosphatemia can develop 1
• Hyperosmolar dehydration may result from excessive magnesium 1

High-Risk Populations

Renal Insufficiency

• Patients with kidney disease should avoid magnesium supplementation due to impaired excretion and high risk of life-threatening hypermagnesemia 2, 5, 3
• The FDA label specifically warns against use in patients with kidney disease 5
• Magnesium should not be used when creatinine clearance is less than 20 mL/min 2

Patients Requiring Caution

• Those with oliguria (particularly pregnant women receiving magnesium sulfate for preeclampsia) are at increased risk for iatrogenic overdose 1
• Patients with metabolic derangements can develop toxicity at relatively lower doses 1
• Individuals on magnesium-restricted diets should consult a physician before use 5

Drug Interactions

• Magnesium can interfere with absorption of certain prescription drugs, particularly tetracycline antibiotics 5, 6
• Multiple drugs including diuretics and proton-pump inhibitors can cause magnesium depletion, creating complex interaction patterns 6
• The FDA label warns patients taking prescription drugs to consult a pharmacist before use 5

Warning Signs Requiring Immediate Medical Attention

Stop Use and Seek Care If:

• Rectal bleeding occurs 5
• No bowel movement after use (may indicate serious condition) 5
• Laxative needed for more than 1 week 5
• Symptoms of toxicity develop: severe weakness, difficulty breathing, marked bradycardia, or altered consciousness 1

Emergency Management of Toxicity

Calcium administration is the antidote for magnesium toxicity and may be lifesaving 1

Specific Treatment Protocol

• Administer calcium chloride 10% solution 5-10 mL IV over 2-5 minutes 1
• Alternatively, use calcium gluconate 10% solution 15-30 mL IV over 2-5 minutes 1
• Empirical calcium should be given immediately in suspected magnesium toxicity during cardiac arrest 1
• Have calcium readily available when administering IV magnesium 2

Common Pitfalls to Avoid

• Do not assume normal serum magnesium excludes toxicity risk—less than 1% of total body magnesium is in blood 2
• Never supplement magnesium without first assessing renal function in at-risk patients 2, 3
• Avoid using magnesium oxide in patients with renal insufficiency despite it being the most commonly prescribed form 2
• Do not ignore early signs like loss of reflexes, as progression to respiratory arrest can be rapid 1
• Recognize that excessive magnesium can paradoxically cause hypomagnesemia through diarrhea-induced losses 4