# Hydralazine: veterinary dose and monograph

> Hydralazine is a veterinary cardiovascular drug. Direct-acting arteriolar VASODILATOR used for afterload reduction — historically valuable in severe/refractory mitral regurgitation with congestive heart failure (it cuts the regurgitant fraction and improves forward output) and in… Indications include severe / refractory mitral regurgitation with CHF — afterload reduction; acute decompensated heart failure (forward-failure) — improve cardiac output; hypertensive crisis / severe systemic hypertension (acute reduction). This monograph lists 5 reference doses for dogs, cats and horses, each with route, frequency and source.

Source: https://vetmasterji.com/drugs/hydralazine · Last reviewed: 2026-10-01

## At a glance

- **Drug class:** Cardiovascular
- **Also known as:** Apresoline, Hydralyn
- **Species with doses:** Dogs, Cats and Horses
- **General dose range:** 0.5–2 mg/kg PO q12h — check the species tables for the indication
- **Routes:** PO, IV
- **Last reviewed:** 1 October 2026

## Indications

- Severe / refractory mitral regurgitation with CHF — afterload reduction
- Acute decompensated heart failure (forward-failure) — improve cardiac output
- Hypertensive crisis / severe systemic hypertension (acute reduction)

## Hydralazine dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

### Dogs

*Hydralazine doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Afterload reduction (refractory MR/CHF) | 0.5–3 mg/kg | PO | q12h | — |
| Hypertensive crisis (acute) | 0.5–3 mg/kg | IV | to effect (in-hospital) | — |
| Hypertensive crisis — CRI | 1.5–5 µg/kg/min | IV | CRI after IV bolus | — |

- Afterload reduction (refractory MR/CHF): Papich 5e: 0.5 mg/kg initially, titrated to 0.5–2 mg/kg q12h PO. Plumb’s p.610 (Kittleson 2007): 1 mg/kg q12h, increasing to 2 then 3 mg/kg if no response — or titrate 1 mg/kg doses 1–2 h apart by blood pressure to a cumulative 3 mg/kg; dogs on ACE inhibitors start at 0.5 mg/kg (severe hypotension risk). Hypertension (5th-step drug): 0.5 mg/kg twice daily.
- Hypertensive crisis (acute): Papich 5e: 0.5–3 mg/kg IV bolus (q12h), or after the initial bolus a CRI of 1.5–5 µg/kg/min. Continuous blood-pressure monitoring.
- Hypertensive crisis — CRI: Papich 5e: 1.5–5 µg/kg/min IV after an initial bolus.

### Cats

*Hydralazine doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Systemic hypertension / afterload reduction | 0.5 mg/kg | PO | q12h | — |

- Systemic hypertension / afterload reduction: Plumb’s p.610: hypertension 0.5 mg/kg PO twice daily as a 4th-step drug (Henik 2007); heart failure start 2.5 mg per cat, up to 10 mg (Kittleson). Papich 5e: 2.5 mg per cat q12–24h PO. Amlodipine is first-line for feline hypertension.

### Horses

*Hydralazine doses for horses*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Afterload reduction (heart failure) | 0.5–1.5 mg/kg | PO | q12h | — |

- Afterload reduction (heart failure): Plumb’s p.610 (Mogg 1999): 0.5 mg/kg IV, then 0.5–1.5 mg/kg PO q12h long term. Papich 5e: 1 mg/kg q12h PO or 0.5 mg/kg IV as needed. ARCI Class 3.

## Contraindications

Hypotension, hypovolaemia, and dynamic outflow obstruction. Use cautiously with other vasodilators/antihypertensives (additive hypotension). Coronary disease (reflex tachycardia increases myocardial oxygen demand). Known hypersensitivity. Avoid abrupt large doses (precipitous hypotension).

## Species-specific warnings

> **Caution:** Cats: Amlodipine is first-line for feline systemic hypertension; reserve hydralazine as an alternative and monitor blood pressure closely.

## Adverse effects

- Hypotension (can be marked — titrate)
- Reflex tachycardia
- Sodium/water retention, RAAS activation
- GI upset, weakness

## Drug interactions

- Other vasodilators / antihypertensives (ACE inhibitors, amlodipine, nitrates): additive hypotension
- NSAIDs: blunt the antihypertensive effect
- Beta-blockers: often combined to control reflex tachycardia
- Sympathomimetics: counteract the effect

## Pregnancy and lactation

Used in human pregnancy hypertension; limited veterinary data — use if maternal benefit is clear, with BP monitoring.

## Monitoring

- Blood pressure (titrate to effect; avoid hypotension)
- Heart rate (reflex tachycardia)
- Renal values and signs of congestion/fluid retention
- Clinical response (forward output, respiratory effort in MR/CHF)

## Toxicity and overdose

Overdose causes profound hypotension, marked reflex tachycardia, and weakness/collapse; chronic high dosing promotes sodium/water retention. Effects are dose-related and manageable by titration.

Management: No specific antidote. IV fluids and, if needed, vasopressors for hypotension; manage reflex tachycardia (a β-blocker, cautiously). Position and support the patient; effects abate as the drug is cleared. Titrate future dosing.

## Clinical pearls

- A potent arteriolar dilator for afterload reduction in severe mitral-regurgitation CHF — improves forward output by cutting the regurgitant fraction
- Expect reflex tachycardia and RAAS activation — usually combined with an ACE inhibitor (± β-blocker) to offset them
- Start low and titrate to blood pressure; hypotension is the main hazard
- Largely replaced by ACE inhibitors/pimobendan for chronic management, but still handy acutely and in hypertensive crisis

## Mechanism of action

A direct arteriolar smooth-muscle relaxant (acting via interference with calcium release/handling in vascular smooth muscle and other incompletely defined pathways), it selectively dilates resistance arterioles to reduce systemic vascular resistance and afterload. In mitral regurgitation this lowers the pressure gradient into the left atrium, reducing the regurgitant fraction and improving forward stroke volume. The reflex sympathetic activation it triggers causes tachycardia and fluid retention.

## Formulations and products

- 25 mg tablet
- 50 mg tablet
- Injection 20 mg/mL
- Apresoline — 25 / 50 mg tablet; 20 mg/mL injection, Tablet / injection (human-label)

## Storage

Tablets/injection: 15–30 °C, protect from light.

## Before you use this dose

> **Caution:** Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

## Other cardiovascular drugs

- [Dobutamine](https://vetmasterji.com/drugs/dobutamine)
- [Dopamine](https://vetmasterji.com/drugs/dopamine)
- [Enalapril](https://vetmasterji.com/drugs/enalapril)
- [Esmolol](https://vetmasterji.com/drugs/esmolol)
- [Metoprolol](https://vetmasterji.com/drugs/metoprolol)
- [Mexiletine](https://vetmasterji.com/drugs/mexiletine)
- [Nitroglycerin (Glyceryl Trinitrate)](https://vetmasterji.com/drugs/nitroglycerin)
- [Norepinephrine (Noradrenaline)](https://vetmasterji.com/drugs/norepinephrine)

## Frequently asked questions

### What is the dose of Hydralazine for dogs?

Reference doses for Hydralazine in dogs: 0.5–3 mg/kg PO q12h (Afterload reduction (refractory MR/CHF)); 0.5–3 mg/kg IV to effect (in-hospital) (Hypertensive crisis (acute)); 1.5–5 µg/kg/min IV CRI after IV bolus (Hypertensive crisis — CRI). Confirm against the label and the patient before use.

### What is the dose of Hydralazine for cats?

Reference doses for Hydralazine in cats: 0.5 mg/kg PO q12h (Systemic hypertension / afterload reduction). Confirm against the label and the patient before use.

### What is the dose of Hydralazine for horses?

Reference doses for Hydralazine in horses: 0.5–1.5 mg/kg PO q12h (Afterload reduction (heart failure)). Confirm against the label and the patient before use.

### When should Hydralazine not be used?

Hypotension, hypovolaemia, and dynamic outflow obstruction. Use cautiously with other vasodilators/antihypertensives (additive hypotension). Coronary disease (reflex tachycardia increases myocardial oxygen demand). Known hypersensitivity. Avoid abrupt large doses (precipitous hypotension).

### What are the side effects of Hydralazine in animals?

Hypotension (can be marked — titrate); Reflex tachycardia; Sodium/water retention, RAAS activation; GI upset, weakness.

## Sources

- Plumb's Veterinary Drug Handbook
- Merck Veterinary Manual, 11th edition
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.