Hydralazine 25 mg is a blood pressure medication that works by relaxing the walls of your blood vessels, allowing blood to flow more easily and reducing the pressure your heart has to pump against. It is FDA-approved for treating essential hypertension (high blood pressure that isn’t caused by another medical condition) and is also widely used off-label to treat heart failure, particularly in combination with another medication. The 25 mg tablet is one of the most commonly prescribed strengths and is often a starting dose.
How Hydralazine Lowers Blood Pressure
Hydralazine belongs to a class of drugs called direct vasodilators. Instead of working through your kidneys or nervous system like many blood pressure medications, it acts directly on the smooth muscle in your artery walls, causing them to relax and widen. Part of how it does this involves binding to iron inside certain enzymes in your blood vessel cells, which disrupts signals that keep those vessels constricted.
Because it targets arteries specifically (rather than veins), your body can respond with a faster heart rate and fluid retention as it tries to compensate for the sudden drop in resistance. This is why hydralazine is almost always prescribed alongside other medications, typically a diuretic to manage fluid buildup and a beta-blocker to keep your heart rate in check. The 2025 AHA/ACC blood pressure guidelines list hydralazine at doses of 100 to 200 mg daily, taken two or three times per day, and specifically note that it should be used with those companion drugs.
Treating High Blood Pressure
Hydralazine is not usually the first medication doctors reach for when treating high blood pressure. More commonly prescribed first-line options include ACE inhibitors, calcium channel blockers, and thiazide diuretics. Hydralazine tends to be added when those medications alone aren’t bringing blood pressure down enough, or when a patient can’t tolerate other drug classes due to side effects or allergies.
It also plays a specific role in managing dangerously high blood pressure during pregnancy. Conditions like preeclampsia and eclampsia require medications that are safe for both the mother and the developing baby, and hydralazine has a long track record of use in these situations. In hypertensive emergencies, where blood pressure spikes to levels that risk organ damage, injectable hydralazine can bring levels down within 10 to 80 minutes.
The Role in Heart Failure
One of the most important uses of hydralazine isn’t on its FDA label at all. When combined with isosorbide dinitrate (a nitrate that relaxes veins), hydralazine becomes a treatment for heart failure with reduced pumping ability. The combination works because hydralazine opens arteries while the nitrate opens veins, reducing the workload on a struggling heart from both directions.
This combination has strong clinical evidence behind it. A large trial of 642 men with impaired heart function found that the hydralazine-nitrate combination reduced the risk of death by 34% at two years compared to placebo. An even more striking result came from the African-American Heart Failure Trial, which enrolled 1,050 patients and was stopped early because the benefit was so clear: a 43% reduction in mortality for patients taking the combination. This particular combination is now a guideline-recommended therapy for Black patients with heart failure, a population that often responds less robustly to ACE inhibitors alone.
Common Side Effects
The most frequently reported side effects stem directly from how hydralazine works. When your arteries suddenly relax, your heart speeds up to maintain blood flow, a phenomenon called reflex tachycardia. You may notice your heart pounding or racing, especially when you first start the medication or after a dose increase. Headaches are also common, caused by the dilation of blood vessels in and around the skull.
Fluid retention is another predictable effect. You might notice swelling in your ankles or feet, or a modest weight gain from water retention. This is one reason doctors typically pair hydralazine with a diuretic. Other side effects can include nausea, dizziness, loss of appetite, and nasal congestion. Most of these tend to improve as your body adjusts over the first few weeks.
Drug-Induced Lupus: A Serious but Reversible Risk
The most notable serious risk with hydralazine is a lupus-like autoimmune reaction. At higher doses, hydralazine can trigger the immune system to produce antibodies that attack your own tissues. Symptoms can include joint pain, muscle aches, fever, skin rashes, and in rarer cases, kidney problems or inflammation affecting multiple organs. Blood tests in affected patients typically show positive antihistone antibodies and elevated antinuclear antibody (ANA) levels.
This reaction is dose-dependent, meaning it is more likely at daily doses above 200 mg, though it can occur at lower doses over long periods. The good news is that drug-induced lupus almost always resolves after stopping hydralazine, unlike the naturally occurring autoimmune disease. If you develop unexplained joint pain, persistent fever, or a new rash while taking this medication, those symptoms warrant prompt attention.
Who Should Avoid Hydralazine
Hydralazine is not appropriate for everyone with high blood pressure. It is contraindicated in people with a dissecting aortic aneurysm, a life-threatening tear in the body’s largest artery where sudden vasodilation could be catastrophic. People with coronary artery disease or other forms of ischemic heart disease should also generally avoid it, because the reflex increase in heart rate raises the heart’s demand for oxygen, which narrowed coronary arteries may not be able to supply.
Taking Hydralazine Effectively
Food has a major impact on how much hydralazine your body actually absorbs. In a study of hypertensive patients, eating a meal right before taking the medication reduced peak blood levels by roughly 69%, and the total amount absorbed dropped by about 44%. This doesn’t necessarily mean you must take it on an empty stomach, but you should take it the same way every time. If you take it with breakfast one day and on an empty stomach the next, you’ll get inconsistent blood pressure control.
The 25 mg tablet is typically taken two to four times daily, depending on your blood pressure response. Your doctor will likely start at a lower total daily dose and increase gradually over several weeks, monitoring how your blood pressure responds. Because the drug’s effect doesn’t last a full 24 hours, splitting doses throughout the day keeps blood pressure more stable than taking it all at once.