[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Thiazide and thiazide-like diuretics](https://durgical.com/en/pharmacology/classes/thiazide-and-thiazide-like-diuretics/) › Metolazone

[Pharmacology](https://durgical.com/en/pharmacology/) · [Thiazide and thiazide-like diuretics](https://durgical.com/en/pharmacology/classes/thiazide-and-thiazide-like-diuretics/)

# Metolazone

A thiazide-like diuretic often added to a loop diuretic when oedema resists treatment. The pair can cause very large fluid and salt losses.

Updated 2026-11-01

- Class: [Thiazide and thiazide-like diuretics](https://durgical.com/en/pharmacology/classes/thiazide-and-thiazide-like-diuretics/)
- Transporter: Na⁺-Cl⁻ cotransporter (NCC) of the distal convoluted tubule
- ATC code: C03BA08

## Mechanism of action

Metolazone, a quinazoline sulfonamide, acts mainly at the cortical diluting site of the distal tubule like the thiazides, blocking sodium and chloride reabsorption. Given with a loop diuretic it blocks the distal sodium reabsorption that rises when the loop is blocked (sequential nephron blockade), producing a much larger diuresis.

- Metolazone blocks sodium reabsorption in the distal tubule
- Alone: moderate thiazide-like diuresis
- With a loop diuretic: two nephron sites blocked in sequence
- Large diuresis in resistant oedema; large K⁺ and Na⁺ losses

## Pharmacokinetics

**Absorption**
Absorbed by mouth; peak levels about eight hours after a dose

**Metabolism**
A small fraction is metabolised

**Elimination**
Mostly unchanged in the urine

## Indications

- Oedema from heart failure, including resistant oedema with a loop diuretic [Heart failure →](https://durgical.com/en/conditions/heart-failure/)
- Oedema from kidney disease [Chronic kidney disease →](https://durgical.com/en/conditions/chronic-kidney-disease/)
- High blood pressure [Arterial hypertension →](https://durgical.com/en/conditions/arterial-hypertension/)

## Adverse effects

### Common

- Hypokalaemia and hyponatraemia
- Dizziness and low blood pressure
- High uric acid and glucose
- Muscle cramps

### Serious (rare, important)

- Rapid-onset severe hyponatraemia or hypokalaemia
- Very large fluid and electrolyte losses with furosemide
- Prerenal azotaemia
- Arrhythmias with digoxin

## Contraindications

- Anuria
- Hepatic coma or precoma
- Hypersensitivity to metolazone

## Key interactions

**Furosemide and other loop diuretics**
Unusually large or prolonged losses of fluid and electrolytes

**Digoxin**
Hypokalaemia increases sensitivity to digoxin; serious arrhythmias

**Neuromuscular blockers**
Hypokalaemia can enhance the block

**Lithium**
Less lithium clearance and risk of toxicity

## What is monitored

- Sodium and potassium, closely when combined with a loop diuretic
- Kidney function
- Weight and fluid balance
- Blood pressure

## Administration and safety

- Formulations of metolazone are not therapeutically equivalent: one product is not swapped for another without the prescriber's review.
- When added to a loop diuretic, the combination is started with close electrolyte and weight monitoring, often in hospital.

## Nursing care

- Before giving: sodium, potassium, kidney function and blood pressure.
- Daily weight; report a large fall in weight or reduced urine output.
- Watch for confusion, weakness, cramps or palpitations.
- Teach: report dizziness, fainting or muscle weakness at once.

## Pregnancy and breastfeeding

Used only for oedema from pathological causes; it crosses the placenta.

## Memory aid

Metolazone is the loop's «sidekick»: alone it is mild, with furosemide it opens the floodgates.

## Test yourself

Think of the answer, then tap the question to see it.

## Why is metolazone added to a loop diuretic in resistant oedema?

Blocking the loop increases sodium reabsorption downstream; metolazone blocks that distal site too, giving a much larger diuresis (sequential nephron blockade).

## What is the main risk of metolazone with furosemide?

Unusually large or prolonged losses of fluid and electrolytes, with severe hyponatraemia or hypokalaemia.

## Why can different metolazone products not be swapped freely?

Its formulations are not therapeutically equivalent, so a change of product can change the effect.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/metolazone/)

## Related drugs

- [Furosemide](https://durgical.com/en/pharmacology/furosemide/)
- [Hydrochlorothiazide](https://durgical.com/en/pharmacology/hydrochlorothiazide/)

## Related conditions

- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/)

[← Back to the class: Thiazide and thiazide-like diuretics](https://durgical.com/en/pharmacology/classes/thiazide-and-thiazide-like-diuretics/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - METOLAZONE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=efa8795e-2c89-4519-b49f-13146fb9ded1) (DailyMed (U.S. National Library of Medicine), 2026); [Metolazone: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682345.html) (MedlinePlus (U.S. National Library of Medicine), 2017); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=C03BA08) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/metolazone/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/metolazone/