Is milk of magnesia (magnesium hydroxide) safe in elderly patients with impaired renal function?

Safety of Milk of Magnesia in Elderly Patients with Impaired Renal Function

Milk of magnesia (magnesium hydroxide) should be used with extreme caution or avoided in elderly patients with impaired renal function due to the significant risk of hypermagnesemia, which can lead to serious cardiovascular complications and death. 1

Risk Assessment

• Saline laxatives containing magnesium (including milk of magnesia) have not been adequately examined in older adults and should be used with caution due to the risk of hypermagnesemia, particularly in those with renal impairment 1
• Elderly patients commonly have declining renal function, which warrants particular caution during administration of renally excreted medications 1
• Fatal cases of hypermagnesemia have been reported in patients taking magnesium hydroxide, especially those with impaired kidney function 2, 3
• Even patients with normal renal function may develop severe hypermagnesemia when taking magnesium hydroxide if they have other risk factors such as bowel obstruction or constipation 4, 5

Pathophysiology of Risk

• Magnesium is primarily excreted by the kidneys, and impaired renal function significantly reduces the body's ability to eliminate excess magnesium 1, 2
• Elderly patients with renal impairment are at particularly high risk as they cannot effectively clear magnesium from the bloodstream 2, 3
• Clinical manifestations of hypermagnesemia include:
  • Cardiovascular: bradycardia, hypotension, cardiac arrest, ECG changes 3
  • Neurological: confusion, lethargy, respiratory depression 2, 4
  • Other: hypothermia, metabolic encephalopathy 2, 3

Safer Alternatives

• Polyethylene glycol (PEG) at 17 g/day is the recommended first-line laxative for elderly patients due to its efficacy and excellent safety profile 1, 6
• Osmotic laxatives (such as PEG, lactulose) or stimulant laxatives (senna, bisacodyl) are generally preferred over magnesium-containing products for elderly patients 1, 6
• For patients with swallowing difficulties or repeated fecal impaction, rectal measures such as suppositories or isotonic saline enemas may be safer alternatives 1

Monitoring Recommendations

• If magnesium hydroxide must be used in elderly patients with mild renal impairment:
  • Regular monitoring of serum magnesium levels is essential 6, 2
  • Monitor for early signs of hypermagnesemia including nausea, flushing, headache, lethargy, and hyporeflexia 2, 3
  • Immediately discontinue magnesium hydroxide if any signs of toxicity appear 2
  • Consider regular assessment of renal function 1

Non-Pharmacological Approaches

• Before considering any medication, implement non-pharmacological measures 6:
  • Ensure adequate toilet access, especially for patients with decreased mobility 1
  • Provide dietetic support and manage decreased food intake 1
  • Optimize toileting habits by educating patients to attempt defecation twice daily, usually 30 minutes after meals 1
  • Encourage adequate fluid intake and physical activity as tolerated 6

Common Pitfalls

• Relying solely on serum creatinine to assess renal function in elderly patients is inadequate; creatinine clearance should be calculated using the Cockcroft-Gault or MDRD equation 1
• Failing to recognize that hypermagnesemia can mimic other conditions such as acute coronary syndrome, septic shock, or cardiogenic shock 3
• Overlooking the risk of hypermagnesemia in patients with normal baseline renal function who develop acute constipation or bowel obstruction 4, 5

In conclusion, given the significant risks and availability of safer alternatives, milk of magnesia should generally be avoided in elderly patients with impaired renal function, with PEG being the preferred alternative due to its proven safety profile and efficacy 1, 6.