Hydralazine: what it treats, how to take it, side effects
Last updated September 3, 2026.
Hydralazine is a vasodilator - it relaxes blood vessels so blood flows more easily through the body. It is one of the oldest blood pressure medicines still in regular use, often added when first-line drugs need a partner.
What does it treat?
High blood pressure. Untreated hypertension damages the brain, heart, blood vessels, and kidneys - damage that shows up as heart disease, heart attack, heart failure, stroke, kidney failure, and vision loss. Medication plus the lifestyle half - low-salt diet, healthy weight, daily exercise, no smoking, moderate alcohol - is what keeps that damage from accumulating.
How do you take it?
A tablet two to four times a day, at the same times every day, exactly as directed. It controls blood pressure but does not cure it, so keep taking it when you feel well and never stop without your doctor. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: flushing, headache, upset stomach, vomiting, loss of appetite, diarrhea, constipation, eye tearing, stuffy nose, and rash.
- Serious - call your doctor immediately: fainting, joint or muscle pain, fever, rapid heartbeat, chest pain, swollen ankles or feet, and numbing or tingling in the hands or feet.
Who should not take it?
Flag any history of heart attack, coronary artery disease, rheumatic heart disease, and heart, kidney, or liver disease - hydralazine speeds up the heart as it dilates vessels, so the heart's condition shapes whether and how it gets used. Tell any surgeon or dentist you take it, and if you become pregnant, call your doctor.
When is it an emergency?
Fainting, chest pain, or a racing heartbeat that will not settle means urgent care. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why do I take hydralazine three times a day when my other pill is once daily?
Hydralazine is short-acting - the vessel relaxation lasts hours, not a full day - so the two-to-four-times-daily schedule is how the coverage stays continuous. It often gets added to a longer-acting drug when one agent alone cannot hold the numbers. The multi-dose rhythm is the price of its specific kind of help.
Why did my doctor ask about my heart before prescribing it?
Because of how it works: dilating vessels drops pressure, and the body often answers with a faster heartbeat - fine for most hearts, a problem for a heart with coronary artery disease or a prior heart attack. MedlinePlus lists heart attack history, coronary artery disease, rheumatic heart disease, and heart, kidney, or liver disease as before-you-start flags. That is also why chest pain and rapid heartbeat sit on the call-immediately list.
What is the lupus-like reaction I have heard about?
Long-term hydralazine can produce a drug-induced lupus-like syndrome - and its early signals are on the serious list: joint or muscle pain with fever. It is uncommon and it typically reverses when the drug stops, but it is exactly why new joint pain plus fever on hydralazine is a call-your-doctor-immediately event, not something to push through.
Why do I feel flushed after my dose?
Flushing is the vasodilation itself - blood vessels opening near the skin. It is the first item on the common-effects list and usually fades as the body adjusts over the first weeks. Flushing that comes with chest pain, fainting, or a racing heart is a different event entirely - that combination is the immediate-call version.
Why can't I stop hydralazine on my own once my numbers look good?
The good numbers are the drug working, not the condition resolving - MedlinePlus says it controls high blood pressure but does not cure it. Stop, and the pressure returns, and the return is silent until it is not. If the regimen feels like too much, that is a dose-review conversation with your prescriber, not a self-directed pause.
What happens if I miss a dose of hydralazine?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. With a multi-dose schedule, anchoring to meals is what keeps the day from slipping; one missed dose is a blip, a pattern of them shows up in the readings.