[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Antiarrhythmics](https://durgical.com/en/pharmacology/classes/antiarrhythmics/) › Quinidine

# Quinidine

## Antiarrhythmics?

Antiarrhythmics

The oldest class IA antiarrhythmic, a relative of quinine. It prolongs the QT, causes diarrhoea and cinchonism, and raises digoxin levels.

Updated Nov 1, 2026

## Quick facts

**Group**
[Antiarrhythmics](https://durgical.com/en/pharmacology/classes/antiarrhythmics/)

**Target**
Cardiac sodium channel Nav1.5 (and potassium channels)

**Used for**
Conversion of atrial fibrillation

**Key effects**
Diarrhoea · Cinchonism · Rash and fever

**Monitor**
QT interval on the ECG · Potassium and magnesium

**ATC**
C01BA01

No doses: the clinician sets them by the current guideline.

On this page

- The basics Mechanism of action
- Indications
- Adverse effects
- Pharmacokinetics
- Memory aid

- Safety Contraindications
- Key interactions
- What is monitored
- Administration and safety
- Pregnancy and breastfeeding
- Nursing care

- Study Test yourself

- Connections Connections

## Mechanism of action

⇅**Cardiac sodium channel Nav1.5 (and potassium channels)**Quinidine

ⓘ The figure shows quinidine in the pore of the human cardiac sodium channel Nav1.5.

**Target in 3D:** Cardiac sodium channel Nav1.5 (and potassium channels). PDB structure 6LQA (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/6LQA) · Drag to rotate · pinch to zoom

Quinidine blocks sodium channels (slower conduction) and several potassium currents (longer repolarisation and QT), the class IA profile. It also has anticholinergic (vagolytic) activity, which can speed AV conduction, alpha-adrenergic blocking (vasodilator) effects and negative inotropy. In malaria it acts as a blood schizonticide.

- Quinidine blocks sodium channels: slower conduction
- It also blocks potassium currents: longer repolarisation and QT
- Its vagolytic effect can speed AV conduction

## Indications

- Conversion of atrial fibrillation or flutter and reducing relapses (now rarely used) [Atrial fibrillation →](https://durgical.com/en/conditions/atrial-fibrillation/)
- Suppression of recurrent life-threatening ventricular arrhythmias [Ventricular tachycardia →](https://durgical.com/en/conditions/ventricular-tachycardia/)
- Life-threatening Plasmodium falciparum malaria (historical intravenous use) [Malaria →](https://durgical.com/en/conditions/malaria/)

## Adverse effects

### Common

- Diarrhoea, nausea and vomiting (very common)
- Cinchonism: tinnitus, hearing loss, vertigo, blurred vision, headache, confusion
- Rash and fever
- Hypotension

### Serious (rare, important)

- QT prolongation and torsades de pointes
- Paradoxical rise in ventricular rate in atrial flutter or fibrillation
- Immune thrombocytopenia and haemolytic anaemia
- Hepatitis, including granulomatous hepatitis
- Higher mortality in trials of non-life-threatening arrhythmias

## Contraindications

- Previous thrombocytopenic purpura with quinidine or quinine
- Complete AV block or junctional or idioventricular rhythm without a pacemaker
- Myasthenia gravis and other conditions harmed by anticholinergic drugs
- Allergy to quinidine

## Key interactions

**Digoxin**
Digoxin levels can double; its dose is usually reduced

**Warfarin**
Stronger anticoagulant effect

**CYP2D6 substrates (for example metoprolol, tricyclics, codeine, flecainide)**
Quinidine blocks their metabolism; codeine converts less to morphine

**Other QT-prolonging drugs**
Higher risk of torsades de pointes

## What is monitored

- QT interval on the ECG (a better safety guide than blood levels)
- Potassium and magnesium
- Platelets, blood count and liver tests
- Digoxin levels when combined

## Administration and safety

- Started where the ECG can be monitored, after correcting low potassium and magnesium.
- Quinidine sulfate and quinidine gluconate are different salts and products: the product is checked.
- Quinidine and quinine are different drugs with similar names: the label is read twice.

## Pregnancy and breastfeeding

Human data are limited; it is used in pregnancy only if clearly needed, for example in life-threatening malaria or arrhythmia.

## Nursing care

- Check pulse and QT; report fainting, palpitations or a fast pulse in atrial flutter.
- Ask about ringing in the ears, hearing or vision changes (cinchonism).
- Watch for diarrhoea and check potassium as ordered; report bruising or petechiae.

## Pharmacokinetics

**Metabolism**
Liver, by CYP3A4, to an active metabolite (3-hydroxyquinidine); it strongly inhibits CYP2D6

**Elimination**
Mostly as metabolites in urine, with part unchanged

## Memory aid

Quinidine Quadruples trouble: QT long, Queasy gut (diarrhoea), Quiet ears (cinchonism) and digoxin up.

## Test yourself

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

## What is cinchonism?

A toxicity of quinidine and quinine: tinnitus, hearing loss, vertigo, blurred vision, headache and confusion, often with gut upset.

## Why can quinidine speed the ventricular rate in atrial flutter?

Its vagolytic effect and slowing of the flutter rate let the AV node conduct more beats.

## How does quinidine change digoxin levels?

It reduces digoxin clearance and volume of distribution, so levels can double.

## Which ECG measure best guides quinidine safety?

The QT interval.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/quinidine/)

[More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology)

## Connections

## Related drugs 4

- [Procainamide](https://durgical.com/en/pharmacology/procainamide/)
- [Digoxin](https://durgical.com/en/pharmacology/digoxin/)
- [Warfarin](https://durgical.com/en/pharmacology/warfarin/)
- [Flecainide](https://durgical.com/en/pharmacology/flecainide/)

## Related conditions 3

- [Atrial fibrillation](https://durgical.com/en/conditions/atrial-fibrillation/)
- [Ventricular tachycardia](https://durgical.com/en/conditions/ventricular-tachycardia/)
- [Malaria](https://durgical.com/en/conditions/malaria/)

[← Back to the class: Antiarrhythmics](https://durgical.com/en/pharmacology/classes/antiarrhythmics/)

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 - QUINIDINE SULFATE- quinidine sulfate tablet tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b0d6341b-d880-4f0b-8844-bc788c03004a) (DailyMed (U.S. National Library of Medicine), 2023); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=C01BA01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); [6LQA: voltage-gated sodium channel Nav1.5 with quinidine](https://www.rcsb.org/structure/6LQA) (RCSB Protein Data Bank, 2021).

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Representative model, for educational purposes only.

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