Hydralazine
Hydralazine HCl
Also known as: Alphapress · Bionobal · Cesoline · Hydrapres · Hyperex · Hyperphen · Ipolina · Nepresol · Novo-Hylazin · Nu-Hydral · Rolazine · Slow-Apresoline · Supres · apressinum · hydralazini · hydrallazine · Hydralazine hydrochloride · Hidralazina
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive therapy in treatment of heart failure (not receiving ACE inhibitors) | 1 mg/kg (starting dose, titrate up to 3 mg/kg if needed) | PO | q12h | — | 🌎 NA |
| Acute, fulminant heart failure due to severe mitral regurgitation (not receiving ACE inhibitors) | 2 mg/kg | PO | Once | — | 🌎 NA |
| Adjunctive therapy in treatment of heart failure (receiving ACE inhibitors) | 0.5 mg/kg (starting dose, increase in 0.5 mg/kg increments to max 3 mg/kg) | PO | q12h | — | 🌎 NA |
| Short-term treatment of CHF secondary to valve disease | Initial dose of 1 mg/kg PO (additional 0.5-1 mg/kg can be administered to max cumulative 3 mg/kg) | PO | q12h | Short-term | 🌎 NA |
| Treatment of systemic hypertension (Fifth step drug) | 0.5 mg/kg | PO | twice daily | — | 🌎 NA |
| Congestive heart failure / Systemic hypertension | 0.5-3 mg/kg | PO | q8-12h | Long-term | 🌍 EU |
- Adjunctive therapy in treatment of heart failure (not receiving ACE inhibitors): Effective dose is 0.5-3 mg/kg PO q12h. Titrate upwards carefully. If BP monitored, can titrate more rapidly in 1-2 hour intervals.
- Acute, fulminant heart failure due to severe mitral regurgitation (not receiving ACE inhibitors): Given along with IV furosemide. May cause hypotension, but benefits outweigh risks.
- Adjunctive therapy in treatment of heart failure (receiving ACE inhibitors): Give with extreme caution as severe hypotension may occur. Blood pressure monitoring required.
- Short-term treatment of CHF secondary to valve disease: Goal MAP is 60-80 mmHg. Once desired BP achieved, administer PO q12h.
- Treatment of systemic hypertension (Fifth step drug): Added if systolic BP > 160 mmHg after enalapril/benazepril, amlodipine (low then high dose), and spironolactone.
- Congestive heart failure / Systemic hypertension: Start at low dose (0.5-1 mg/kg q12h), monitor blood pressure regularly and increase to 2 mg/kg or even 3 mg/kg q12h if necessary.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive therapy in treatment of heart failure | start titration at 2.5 mg (total dose) and if necessary, increase up to 10 mg | PO | q12h | — | 🌎 NA |
| Treatment of systemic hypertension (Fourth step drug) | 0.5 mg/kg | PO | twice daily | — | 🌎 NA |
| Systemic hypertension | 2.5-10 mg/cat | PO | q12h | Long-term | 🌍 EU |
- Adjunctive therapy in treatment of heart failure: Follow similar titration protocols as dogs.
- Treatment of systemic hypertension (Fourth step drug): Added if systolic BP > 160 mmHg after amlodipine, ACE inhibitor, and spironolactone.
- Systemic hypertension: Start at low dose and titrate upwards cautiously. Monitor blood pressure regularly.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive therapy in treatment of heart failure (afterload reducer) | 0.5 mg/kg | IV | Once | — | 🌎 NA |
| Adjunctive therapy in treatment of heart failure (long-term therapy) | 0.5-1.5 mg/kg | PO | q12h | Long-term | 🌎 NA |
- Adjunctive therapy in treatment of heart failure (afterload reducer): ARCI UCGFS Class 3 Drug.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Hydralazine is a potent, direct-acting arteriolar vasodilator used primarily for the management of systemic hypertension and as an afterload reducer in the adjunctive treatment of congestive heart failure (CHF) in small animals.
- Clinical Utility: Particularly beneficial in cases of mitral valve insufficiency, large septal defects, or severe aortic regurgitation where reducing the resistance against which the heart pumps (afterload) is critical.
- Refractory CHF: Often utilized when ACE inhibitors (like enalapril) are insufficient to achieve clinical improvement in dogs with mitral insufficiency.
- Combination Therapy: Frequently combined with diuretics to counteract its tendency to cause sodium and water retention, and sometimes with beta-blockers to blunt reflex tachycardia.
- Limitations: Not particularly useful in treating heart failure when primary myocardial disease (e.g., dilated cardiomyopathy) is the sole issue without significant valvular regurgitation.
Mechanism of action
Hydralazine acts as a semicarbazide-sensitive amine oxidase (SSAO) inhibitor, directly relaxing vascular smooth muscle.
- Mechanism: Alters cellular calcium metabolism in smooth muscle → interferes with calcium movements → prevents the initiation and maintenance of the contractile state.
- Vascular Selectivity: Exerts a much greater effect on arterioles than on veins. This selective arterial dilation significantly decreases systemic vascular resistance (SVR) and blood pressure.
- Cardiac Effects: In CHF patients, the reduction in afterload → significantly increases forward cardiac output.
- Compensatory Responses: In non-CHF hypertensive patients, the sudden drop in blood pressure can trigger baroreceptor-mediated reflex tachycardia and activation of the renin-angiotensin-aldosterone system (RAAS) → leading to sodium and water retention.
Safety & warnings
Contraindications
- Known hypersensitivity to hydralazine
- Coronary artery disease
- Hypovolemia
- Preexisting hypotension
- Hypovolaemia
- Renal impairment
- Cerebral bleeding
Adverse effects
- Weakness and lethargy
- Syncope (fainting)
- Reflex tachycardia
- Sodium and water retention
- GI distress (vomiting, diarrhea)
- Increased creatinine levels
- SLE-like syndrome (documented in humans, theoretical in animals)
- Lacrimation and conjunctivitis
- Peripheral neuritis
- Blood dyscrasias
- Urinary retention
- Constipation
- Severe hypotension
- Anorexia (especially in cats)
- Vomiting (especially in cats)
Precautions
Dose Titration: Doses must be titrated upwards carefully to avoid severe, life-threatening hypotension.
- Renal Disease: Use with caution. Reduced renal blood flow can activate the RAAS and exacerbate renal injury; pretreatment with an ACE inhibitor and spironolactone is often advised.
- Intracerebral Bleeding: Use with caution due to potential effects on cerebral blood flow.
- Autoimmune Disease: Use cautiously in patients with preexisting autoimmune diseases, as an SLE-like syndrome has been documented in humans.
- Pregnancy: FDA Category C (humans); Papich Class B (veterinary). Safe for use if used cautiously, but crosses the placenta. Excreted in milk but generally considered compatible with nursing.
Drug interactions
May cause additive hypotensive effect; usually used for therapeutic advantage
May cause additive hypotensive effect; usually used for therapeutic advantage
Potentially could cause profound hypotension
May cause additive hypotensive effect; usually used for therapeutic advantage
Hydralazine may increase furosemide's renal effects
May cause additive hypotensive effect
Hydralazine may cause decreased pressor effect and may cause additive tachycardia
Enhanced hypotensive effects
Enhanced hypotensive effects
Enhanced hypotensive effects
Enhanced hypotensive effects
Enhanced hypotensive effects
Enhanced hypotensive effects
May cause tachycardia
Reduced pressor response to adrenaline
Monitoring
- Baseline thoracic radiographs
- Mucous membrane color and capillary refill time
- Serum electrolytes and creatinine
- Arterial blood pressure (Target MAP 60-80 mmHg for short-term CHF treatment)
- Venous PO2
- Occasional CBC (due to possibility of blood dyscrasias)
- Systemic blood pressure (frequent monitoring during initiation)
- Heart rate and rhythm
- Renal function (BUN, Creatinine)
- Hydration status and body weight (to monitor for fluid retention)
Pharmacokinetics
Half-life
Absorption
Rapidly absorbed after oral administration. Onset of action within 1 hour, peak effects at 3-5 hours. High first-pass effect. Food decreases oral bioavailability in dogs by about 63% (though some reports suggest food may enhance tablet bioavailability).
Distribution
Widely distributed in body tissues. Approximately 85% bound to plasma proteins in humans. Crosses the placenta and very small amounts are excreted into milk.
Metabolism
Extensively metabolized in the liver. N-acetylation is a primary enzymatic pathway, which is mostly absent in dogs, leading to increased risks for toxicity.
Elimination
Approximately 15% is excreted unchanged in the urine. Duration of action in dogs after oral administration is reportedly 11-13 hours.
Overdose
Clinical Signs: Overdoses are characterized by severe hypotension, reflex tachycardia or other arrhythmias, skin flushing, and myocardial ischemia.
Treatment:
- Decontamination: Evacuate gastric contents and administer activated charcoal using standard precautions if ingestion was recent and cardiovascular status is stable.
- Cardiovascular Support: Treat shock using volume expanders. Avoid pressor agents if possible.
- Pressor Agents: If required to maintain blood pressure, use a minimally arrhythmogenic agent (e.g., phenylephrine or methoxamine).
- Additional Support: Digitalis agents may be required. Diligently monitor blood pressure and renal function.
Available products
Formulations
- Oral tablets
- Injection
- 25 mg oral tablet
- 50 mg oral tablet
Human-labeled
- Hydralazine HCl Oral Tablets: 10 mg, 25 mg, 50 mg & 100 mg (Apresoline, generic)
- Hydralazine Injection: 20 mg/mL in 1 mL vials (generic)
- Apresoline 25 mg tablets
- Apresoline 50 mg tablets
Regulatory status
POM (Prescription Only Medicine). Used under the cascade.
POM (Prescription Only Medicine). Used under the prescribing cascade.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store tablets in tight, light-resistant containers at room temperature. Store injectable product at room temperature; avoid refrigeration or freezing. When mixed with infusion solutions, a color change may occur over 8-12 hours, which does not necessarily indicate a loss in potency.
Client information
Hydralazine is a medication used to lower blood pressure and reduce the workload on your pet's heart, particularly if they have leaky heart valves.
- Strict Compliance: It is crucial to give this medication exactly as prescribed by your veterinarian to maximize its benefits and avoid dangerous drops in blood pressure.
- What to Watch For: Contact your veterinarian immediately if your pet becomes unusually lethargic, weak, depressed, or if they faint. These can be signs that their blood pressure has dropped too low (hypotension).
- Side Effects: You may notice mild vomiting or diarrhea. If these persist, inform your vet.
- Diet: Ask your veterinarian whether to give this medication with or without food, as food can affect how much drug is absorbed into your pet's system.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.