Hydralazine is a blood pressure medication that works by relaxing the walls of your arteries, allowing blood to flow more easily. It’s used to treat high blood pressure (hypertension), heart failure, and dangerously high blood pressure during pregnancy. While newer blood pressure drugs have largely replaced it as a first-choice treatment, hydralazine remains an important option in specific situations where other medications aren’t enough or can’t be used.
How Hydralazine Lowers Blood Pressure
Hydralazine is a direct vasodilator, meaning it acts on the smooth muscle cells lining your artery walls rather than working through your nervous system or kidneys like many other blood pressure drugs. Inside those muscle cells, it blocks the release of calcium from internal storage sites. Since calcium is what triggers muscle contraction, less calcium means the artery walls relax and widen. This reduces the resistance your heart has to pump against, which brings blood pressure down.
One important detail: hydralazine works almost exclusively on arteries, not veins. This selective effect is part of why it causes a specific set of side effects, which we’ll get to below.
Treating High Blood Pressure
Hydralazine is FDA-approved for essential hypertension, the most common form of high blood pressure. In current practice, it’s typically reserved for cases where blood pressure hasn’t responded well to other medications. It’s not usually the first drug prescribed because it needs to be taken multiple times a day and tends to cause more noticeable side effects than newer options.
When taken by mouth, hydralazine starts lowering blood pressure within 20 to 30 minutes, with levels peaking in your bloodstream at about one to two hours. The effect of each dose lasts roughly two to four hours, which is why it’s often taken three or four times daily. Your body’s ability to process the drug varies based on genetics. Some people break it down slowly (“slow acetylators”), absorbing about 50% of each dose, while others process it quickly (“fast acetylators”) and absorb only about 30%. This genetic variation means two people on the same dose can have noticeably different responses.
Food affects absorption in a complicated way. Eating actually increases how much hydralazine your gut absorbs, but chemical changes during digestion can reduce the drug’s effectiveness in your bloodstream. For this reason, it helps to take each dose at the same time relative to meals so the effect stays consistent.
Heart Failure Treatment
Hydralazine plays a distinct role in heart failure, but it’s always used alongside another medication called isosorbide dinitrate. While hydralazine opens arteries, isosorbide dinitrate opens veins. Together, they reduce the workload on a weakened heart from both sides of the circulation.
The American Heart Association gives this combination its strongest recommendation (Class I) for Black patients with symptomatic heart failure who already take standard therapy. This recommendation comes from a landmark trial called A-HeFT, which enrolled Black patients with moderate to severe heart failure symptoms and reduced heart pumping ability. The trial found significant benefits from adding the combination to standard treatment. For non-Black patients, the recommendation is somewhat weaker (Class IIA), and for people who can’t tolerate other common heart failure drugs like ACE inhibitors, it carries a Class IIB recommendation, meaning it may be considered.
Blood Pressure Emergencies in Pregnancy
Hydralazine given intravenously is considered a first-line treatment for severe, sudden-onset high blood pressure during pregnancy and the postpartum period. This includes dangerously elevated blood pressure in preeclampsia and eclampsia, conditions that threaten both the mother and baby.
The IV form works faster than pills, with blood pressure dropping within 5 to 30 minutes of injection. In hospital settings, doses are typically given every 10 minutes as needed until blood pressure reaches a safe range. The effects of a single IV dose can last anywhere from two to six hours, and sometimes up to 12 hours because the drug binds tightly to artery walls. Hydralazine has decades of safety data in pregnancy, which is one reason it remains a go-to choice even as other options have become available.
Side Effects to Know About
Because hydralazine relaxes arteries but not veins, your body interprets the sudden drop in arterial pressure as a signal that something is wrong. The nervous system responds by speeding up the heart, a reaction called reflex tachycardia. You might feel your heart racing or pounding, especially in the first days of treatment. This compensatory response can also cause your body to retain fluid, leading to swelling in the ankles or feet. To counteract these effects, hydralazine is often prescribed alongside a beta-blocker (to slow the heart rate back down) and a diuretic (to prevent fluid buildup).
A rarer but more serious concern with long-term use is a condition called drug-induced lupus. This autoimmune reaction can cause joint pain, skin rashes, and fever. It’s more common at higher doses and in slow acetylators, who maintain higher drug levels in their blood. The condition typically resolves after stopping the medication, but it’s one reason doctors monitor patients on hydralazine over time.
How It Compares to Other Blood Pressure Drugs
Hydralazine has been around since the 1950s, and the landscape of blood pressure treatment has changed dramatically since then. ACE inhibitors, calcium channel blockers, and similar drugs are now preferred for most people because they can be taken once or twice daily, cause fewer reflexive side effects, and have stronger evidence for preventing long-term complications like heart attacks and strokes.
Where hydralazine still holds its ground is in the situations described above: resistant hypertension that hasn’t budged with other drugs, heart failure in combination with isosorbide dinitrate, and acute blood pressure crises during pregnancy. It’s also useful for people who can’t tolerate other classes of medication due to allergies or side effects. Rather than a first-line drug, think of hydralazine as a specialist, called in when the more common options aren’t doing the job.