Hydralazine is a blood pressure medication that works by relaxing the walls of your blood vessels. It’s prescribed for high blood pressure (hypertension), heart failure, and dangerously high blood pressure during pregnancy. While newer medications have largely replaced it as a first-choice option for routine hypertension, hydralazine remains an important tool in specific clinical situations where other drugs aren’t suitable or fast-acting enough.
How Hydralazine Lowers Blood Pressure
Hydralazine is a direct vasodilator, meaning it acts on the smooth muscle cells lining your arteries and causes them to relax. When those muscles relax, the blood vessels widen, blood flows more easily, and pressure drops. The drug accomplishes this by interfering with the chemical process that muscle cells need in order to contract. Specifically, it blocks a key step in how calcium triggers muscle tightening inside artery walls. Without that signal, the vessels stay looser and more open.
This effect is selective. Hydralazine primarily relaxes arteries rather than veins, which is why it’s effective at reducing the resistance your heart pumps against but can also trigger some predictable side effects (more on those below).
High Blood Pressure
Hydralazine is FDA-approved for essential hypertension, the most common type of high blood pressure with no single identifiable cause. However, in the 2025 joint guidelines from the American Heart Association and American College of Cardiology, it sits well below the first-line options. Doctors typically reach for it only after diuretics, ACE inhibitors, calcium channel blockers, and beta-blockers have been tried or ruled out.
When taken by mouth, the typical adult dose ranges from 40 to 200 mg per day, split into two to four doses. Most people start at 10 mg four times a day, with gradual increases. The blood-pressure-lowering effect kicks in within 20 to 30 minutes of swallowing a tablet and lasts roughly 2 to 4 hours, which is why multiple daily doses are necessary.
Because hydralazine can cause the body to retain sodium and water, and because the sudden drop in arterial pressure can make the heart beat faster (a reflex response), it’s almost always prescribed alongside a diuretic and a beta-blocker to counteract those effects.
Heart Failure in Black Patients
One of hydralazine’s most distinctive roles is in treating heart failure, specifically in a fixed-dose combination tablet with isosorbide dinitrate (a nitrate that relaxes veins). This combination was approved after clinical trials showed it significantly reduced hospitalizations and deaths in self-identified Black patients with heart failure who were already on standard therapy.
The standard tablet contains 37.5 mg of hydralazine and 20 mg of isosorbide dinitrate, taken three times daily. The two drugs complement each other: hydralazine opens arteries (reducing the workload on the heart), while the nitrate opens veins (reducing the volume of blood returning to the heart). Together, they improve how efficiently the heart pumps.
Severe Hypertension During Pregnancy
Intravenous hydralazine has long been considered a first-line treatment for acute, severe hypertension in pregnant and postpartum women. This includes conditions like preeclampsia, where blood pressure can spike to dangerous levels and raise the risk of stroke. Treatment guidelines call for action within 30 to 60 minutes of confirmed severe hypertension to protect the mother.
When given by IV, hydralazine starts working within 5 to 20 minutes and reaches its peak effect somewhere between 10 and 80 minutes. The blood-pressure-lowering effect lasts 2 to 6 hours. Its long track record of use in pregnancy is one reason it remains a go-to option, even as newer medications have gained ground in other settings. If hydralazine and other first-line agents fail to control the crisis, a specialist team is brought in to manage the situation.
IV Use for Hypertensive Urgency
Outside of pregnancy, injectable hydralazine is FDA-approved for severe essential hypertension when a patient can’t take pills or when blood pressure needs to come down quickly. This typically happens in a hospital setting. The injectable form can be given into a vein or into a muscle, with the IV route working fastest (onset in 5 to 20 minutes) and the intramuscular route taking 10 to 30 minutes to take effect.
Side Effects and Drug-Induced Lupus
The most common side effects of hydralazine stem directly from how it works. Widening the arteries can cause headaches, a rapid heartbeat, flushing, and dizziness. Fluid retention, nasal congestion, and nausea are also reported. Many of these effects are manageable, especially when the drug is paired with a beta-blocker and diuretic.
The side effect that gets the most attention is drug-induced lupus, an autoimmune-like reaction that can develop after taking hydralazine for at least 3 to 6 months. Symptoms include joint pain and swelling, fever, general malaise, loss of appetite, chest pain that worsens with deep breaths, and a skin rash on sun-exposed areas. The risk increases at higher doses, which is one reason the 2025 AHA/ACC guidelines specifically flag the lupus-like syndrome as a concern at the upper end of the dosing range. In rare cases, kidney inflammation can occur. The good news is that drug-induced lupus typically resolves after the medication is stopped.
Where Hydralazine Fits Today
Hydralazine has been around since the 1950s, and it’s no longer a first-choice medication for most people with high blood pressure. Newer drug classes tend to be taken once daily, have fewer reflex side effects, and offer additional benefits like kidney protection. But hydralazine fills roles that newer drugs don’t. It’s one of the few blood pressure medications with a strong safety record in pregnancy. Its combination with isosorbide dinitrate addresses a specific gap in heart failure treatment. And its injectable form remains useful for blood pressure emergencies when rapid, reliable results are needed.
For people who can’t tolerate ACE inhibitors, angiotensin receptor blockers, or calcium channel blockers due to allergies, kidney problems, or other issues, hydralazine provides a proven alternative. It’s a niche medication now, but an important one in the situations where it’s called for.