Why do I experience fatigue after taking milk of magnesia (magnesium hydroxide)?

Fatigue After Taking Milk of Magnesia

Extreme fatigue after taking milk of magnesia is not a recognized direct side effect of the medication itself, but rather may indicate either electrolyte disturbances (particularly hypomagnesemia from excessive diarrhea, or hypermagnesemia in those with kidney dysfunction) or dehydration from the laxative effect.

Understanding the Mechanism

Milk of magnesia (magnesium hydroxide) works as an osmotic laxative and is recommended as a first-line, inexpensive treatment option for constipation 1. However, the medication can cause several physiological changes that may lead to fatigue:

Electrolyte Disturbances

The most critical concern is that magnesium-containing laxatives can paradoxically cause both hypermagnesemia and hypomagnesemia, both of which produce fatigue:

• Hypermagnesemia (excessive magnesium) can occur, particularly in patients with any degree of kidney dysfunction, causing progressive symptoms including muscular weakness, drowsiness, confusion, and depressed level of consciousness 2, 3
• Paradoxical hypomagnesemia can develop when milk of magnesia causes severe diarrhea, leading to excessive magnesium loss through the gastrointestinal tract, which manifests as weakness and fatigue 4

Dehydration and Volume Depletion

• Osmotic laxatives like milk of magnesia draw water into the intestines, which can lead to significant fluid losses if diarrhea develops 1
• Volume depletion causes fatigue, weakness, and can trigger secondary hyperaldosteronism, which further worsens electrolyte imbalances 2

Immediate Assessment Required

You should check the following immediately:

• Serum magnesium level - critical to determine if you have hyper- or hypomagnesemia 2
• Kidney function (creatinine, eGFR) - even mild renal insufficiency increases risk of magnesium toxicity 1, 3
• Other electrolytes (potassium, calcium) - hypomagnesemia causes refractory hypokalemia and hypocalcemia 2
• Hydration status - assess for signs of volume depletion from excessive laxative effect 2

When to Seek Emergency Care

Seek immediate medical attention if you experience:

• Severe weakness or inability to move
• Confusion or altered mental status
• Irregular heartbeat or chest pain
• Difficulty breathing
• Severe or persistent diarrhea

These symptoms may indicate severe hypermagnesemia, which can cause bradycardia, cardiac arrhythmias, respiratory depression, and cardiovascular collapse 2, 3.

Management Recommendations

If Continuing Milk of Magnesia

• Start at a lower dose and increase only if necessary 1
• Avoid use entirely if you have any kidney problems - this is the most critical contraindication 1, 3
• Ensure adequate hydration - drink plenty of water to prevent volume depletion 2
• Take at night when intestinal transit is slowest to minimize excessive diarrhea 2

Alternative Laxative Options

Consider switching to polyethylene glycol (PEG), which is strongly recommended as first-line therapy and does not carry the same electrolyte risks 1. PEG has been shown to be equally or more effective than milk of magnesia with better tolerability and no risk of magnesium-related complications 5.

Common Pitfalls to Avoid

• Never assume fatigue is "normal" with laxative use - it warrants electrolyte evaluation 2
• Do not take milk of magnesia long-term without medical supervision, especially if you have any kidney issues 1
• Avoid excessive dosing - more is not better and can lead to dangerous electrolyte imbalances 4
• Do not ignore persistent diarrhea - this can cause paradoxical hypomagnesemia even from a magnesium-containing product 4

Bottom Line

Stop taking milk of magnesia and have your electrolytes and kidney function checked. The fatigue you're experiencing is not a typical side effect and suggests either electrolyte disturbance or dehydration that requires medical evaluation. Switch to PEG (polyethylene glycol) as a safer alternative that does not carry these risks 1, 5.