What are the side effects of metolazone?

Metolazone Side Effects

Metolazone causes severe electrolyte disturbances—particularly hypokalemia, hyponatremia, and metabolic alkalosis—in approximately 10% of treatment episodes, along with worsening renal function, and these adverse effects occur more frequently than with high-dose loop diuretics alone. 1, 2, 3

Common Electrolyte and Metabolic Disturbances

• Hypokalemia is the most frequent electrolyte abnormality, occurring in a substantial proportion of patients receiving metolazone, especially when combined with loop diuretics 1, 2, 3
• Hyponatremia develops commonly during metolazone therapy; clinically important hyponatremia (<125 mmol/L) occurs in approximately 10% of treatment episodes 1, 2, 3
• Hypochloremic alkalosis results from excessive chloride loss in the distal tubule 1
• Hyperglycemia and glycosuria can occur, particularly in patients with diabetes or pre-diabetes 1
• Hyperuricemia is common and may precipitate acute gouty attacks in susceptible patients 1, 4
• Hypercalcemia may develop during prolonged therapy 1
• Hypomagnesemia and hypophosphatemia occur but are less frequently monitored 1

Renal Effects

• Worsening renal function is strongly associated with metolazone use, manifesting as increases in serum creatinine and blood urea nitrogen (BUN) 1, 2, 4, 3
• Initial treatment produces small increases in serum creatinine secondary to diuresis-induced volume depletion, which may be acceptable if the patient remains asymptomatic 4
• Azotemia from excessive volume depletion is a significant risk, particularly when metolazone is combined with loop diuretics 5

Cardiovascular and Hemodynamic Effects

• Orthostatic hypotension occurs frequently due to excessive volume depletion 1
• Excessive volume depletion and hemoconcentration can lead to prerenal azotemia and tissue hypoperfusion 1
• Venous thrombosis has been reported, likely related to hemoconcentration 1
• Chest pain/discomfort and palpitations may occur 1

Severe Dermatologic and Hypersensitivity Reactions

• Toxic epidermal necrolysis (TEN) and Stevens-Johnson Syndrome are rare but life-threatening reactions 1
• Necrotizing angiitis (cutaneous vasculitis) and skin necrosis represent severe hypersensitivity phenomena 1
• Photosensitivity dermatitis is a common, less severe skin reaction 1
• Urticaria, pruritus, purpura, and petechiae occur with variable frequency 1

Hematologic Complications

• Aplastic/hypoplastic anemia is a rare but serious adverse effect 1
• Agranulocytosis, leukopenia, and thrombocytopenia can occur and require immediate discontinuation 1

Gastrointestinal Effects

• Hepatitis and intrahepatic cholestatic jaundice are serious hepatic complications 1
• Pancreatitis is a rare but potentially life-threatening adverse effect 1
• Nausea, vomiting, epigastric distress, diarrhea, and constipation are common gastrointestinal complaints 1
• Anorexia and abdominal bloating/pain may occur 1

Neurologic and Psychiatric Effects

• Syncope can result from severe volume depletion or electrolyte disturbances 1
• Neuropathy and paresthesias have been reported 1
• Psychotic depression is a rare but serious psychiatric complication 1
• Dizziness/lightheadedness, drowsiness, fatigue, and weakness are common complaints 1
• Vertigo and headache occur with variable frequency 1
• Restlessness resulting in insomnia may develop 1

Musculoskeletal Effects

• Acute gouty attacks are precipitated by hyperuricemia 1, 4
• Muscle cramps or spasm occur frequently, particularly with severe electrolyte depletion 1
• Joint pain has been reported 1

Other Adverse Effects

• Transient blurred vision may occur 1
• Impotence has been reported in male patients 1
• Dry mouth and chills are less common complaints 1

Comparative Risk: Metolazone vs. High-Dose Loop Diuretics

• Metolazone is independently associated with increased mortality after controlling for baseline characteristics and propensity to receive the drug (hazard ratio 1.20,95% CI 1.04-1.39) 3
• High-dose loop diuretics are NOT associated with increased mortality under the same experimental conditions (hazard ratio 0.97 per 100 mg IV furosemide, 95% CI 0.90-1.06) 3
• Metolazone shows stronger associations with hyponatremia, hypokalemia, and worsening renal function compared to high-dose loop diuretics, even after propensity adjustment 3

Critical Monitoring Requirements

• Check serum potassium, sodium, and creatinine 1-2 days after initiating metolazone, then every 3-7 days during active treatment 5
• Monitor daily weights targeting 0.5-1.0 kg loss per day 5
• Assess for signs of excessive diuresis: hypotension, dizziness, oliguria, and marked fatigue 5
• Watch for severe hyponatremia (sodium <125 mmol/L) or severe hypokalemia (potassium <3.0 mmol/L), which require immediate drug cessation 5

When to Reduce or Discontinue Metolazone

• Moderate or severe adverse reactions warrant dose reduction or therapy withdrawal 1
• Severe hyponatremia (<120-125 mmol/L) requires immediate discontinuation 5
• Severe hypokalemia (<3.0 mmol/L) mandates stopping the drug 5
• Progressive renal failure despite adequate diuresis necessitates reassessment 5
• Anuria is an absolute contraindication to continued use 5