Hydralazine HCl

What is this medication for?

• Afterload reducer in CHF (esp. mitral valve insufficiency when ACE inhibitors insufficient; severe aortic regurgitation, large septal defects); systemic high blood pressure
• Same — adjunctive CHF/high blood pressure

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

• Always pair with a diuretic in CHF (sodium/water retention otherwise undoes the benefit) and consider pre-treating kidneys with ACEi + spironolactone.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Hypotension, weakness, syncope (Most prevalent — especially with rapid up-titration)
• Reflex unusually fast heart rate (Expected; treat with digoxin or beta-blocker (caution: β-blockers ↓ cardiac performance))
• Sodium/water retention (RAAS activation) (If not paired with diuretic; can ↑ creatinine)
• Vomiting/diarrhea (stomach/intestinal distress)
• SLE-like syndrome, blood dyscrasias, peripheral neuritis (Human-documented; caution with autoimmune disease)

Worth a phone call to your veterinarian

• Any new symptom that worries you.

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Hypersensitivity; coronary artery disease; hypovolemia or preexisting low blood pressure
• has or has had: Renal disease (RAAS activation + reduced renal flow — pre-treat with ACE inhibitor ± spironolactone); intracerebral bleeding; preexisting autoimmune disease; not useful when primary myocardial disease present

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Diuretics
  • Beta-blockers
  • Other hypotensives/ACE inhibitors
  • Nitroglycerin ointment

Storage and handling

Room temperature; protect from moisture.

Good to know

• Titrate slowly — severe low blood pressure is the classic hydralazine error. When BP monitoring is unavailable, titrate on clinical + radiographic signs over 12–48 h steps.
• Always pair with a diuretic in CHF (sodium/water retention otherwise undoes the benefit) and consider pre-treating kidneys with ACEi + spironolactone.
• Dogs lack the N-acetylation pathway that humans use to clear hydralazine — narrower safety margin than human pharmacology suggests.
• Reserved mostly for mitral-insufficiency CHF unresponsive to ACE inhibitors — not effective for primary myocardial failure.