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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice.
Nitrate Vasodilator [EPC]
Hydralazine is a potent peripheral vasodilator used primarily to treat essential hypertension and heart failure. It works by relaxing vascular smooth muscle to lower blood pressure and reduce cardiac afterload.
Name
Hydralazine
Raw Name
HYDRALAZINE HYDROCHLORIDE
Category
Nitrate Vasodilator [EPC]
Salt Form
Hydrochloride
Drug Count
5
Variant Count
161
Last Verified
February 17, 2026
RxCUI
905222, 966571, 905199, 905225, 905395, 905377, 905379
UNII
FD171B778Y, IA7306519N
About Hydralazine
Hydralazine is a potent peripheral vasodilator used primarily to treat essential hypertension and heart failure. It works by relaxing vascular smooth muscle to lower blood pressure and reduce cardiac afterload.
Detailed information about Hydralazine
References used for this content
This page is for informational purposes only and does not replace medical advice. Consult a qualified healthcare professional before using any medication containing Hydralazine.
Historically, hydralazine was one of the first oral antihypertensive medications available in the United States. While the advent of newer classes like ACE inhibitors and Calcium Channel Blockers moved hydralazine to a third-line or fourth-line therapy for routine hypertension, it remains a critical tool for specific patient populations. It is particularly notable for its role in treating resistant hypertension and its documented efficacy in managing heart failure, especially when used in combination with nitrates. Healthcare providers often turn to hydralazine when standard therapies are insufficient or contraindicated, making it a vital 'rescue' or 'add-on' medication in complex cardiovascular regimens.
The precise molecular mechanism of hydralazine is complex and involves multiple pathways, though its primary effect is the relaxation of vascular smooth muscle. At the cellular level, hydralazine is thought to interfere with the movement of calcium ions (Ca2+) within the smooth muscle cells of the arteries. Calcium is essential for muscle contraction; by inhibiting its release from the sarcoplasmic reticulum or blocking its entry into the cell, hydralazine prevents the muscle from tightening.
Furthermore, hydralazine is believed to stimulate the release of nitric oxide from the vascular endothelium and may also inhibit the enzyme responsible for breaking down nitric oxide. This increase in nitric oxide levels leads to the activation of guanylate cyclase, which increases levels of cyclic guanosine monophosphate (cGMP), a potent signaling molecule that induces vasodilation. A unique characteristic of hydralazine is its selectivity: it acts almost exclusively on arterioles (small arteries) rather than veins. This selective arterial dilation reduces systemic vascular resistance without causing the significant venous pooling associated with other types of vasodilators. This reduction in afterload is particularly beneficial for patients with heart failure, as it allows the heart to pump blood more efficiently against lower resistance.
Understanding how the body processes hydralazine is essential for safe and effective dosing, particularly because individual metabolism can vary significantly.
Hydralazine is FDA-approved for several specific indications, and its use is strictly guided by clinical guidelines:
Hydralazine is available in multiple formulations to suit different clinical needs:
> Important: Only your healthcare provider can determine if Hydralazine is right for your specific condition. This medication requires careful monitoring and is usually part of a multi-drug regimen.
The dosage of hydralazine is highly individualized and must be 'titrated' (gradually adjusted) by a healthcare provider to achieve the desired blood pressure effect while minimizing side effects.
Hydralazine is used in children, but only under the strict supervision of a pediatric cardiologist or nephrologist.
In patients with moderate to severe kidney disease (renal impairment), hydralazine can accumulate in the body. Healthcare providers may reduce the frequency of dosing (e.g., once or twice daily instead of four times) or lower the total daily dose to prevent toxicity. Regular monitoring of serum creatinine and blood pressure is mandatory.
Because hydralazine is extensively metabolized by the liver, patients with liver disease may experience higher-than-normal blood levels of the drug. Dose reductions are often necessary to avoid excessive hypotension and other systemic side effects.
Older adults are generally more sensitive to the blood-pressure-lowering effects of hydralazine. There is an increased risk of 'orthostatic hypotension' (dizziness when standing up), which can lead to falls. Doctors typically start with the lowest possible dose and increase it very slowly.
To get the most benefit from hydralazine, it must be taken exactly as prescribed:
If you miss a dose, take it as soon as you remember. However, if it is almost time for your next scheduled dose, skip the missed dose and return to your regular schedule. Never double the dose to catch up, as this can cause a dangerous drop in blood pressure.
An overdose of hydralazine is a medical emergency. Signs include extreme low blood pressure (hypotension), a very fast or irregular heartbeat (tachycardia), headache, flushing, and potentially shock. If an overdose is suspected, contact a poison control center or seek emergency medical care immediately. Treatment usually involves IV fluids to support blood pressure and cardiovascular monitoring.
> Important: Follow your healthcare provider's dosing instructions exactly. Do not adjust your dose or stop taking this medication without medical guidance, as sudden discontinuation can cause 'rebound' high blood pressure.
Hydralazine is a potent medication, and many patients experience side effects, particularly when starting therapy or increasing the dose. Common reactions include:
> Warning: Stop taking Hydralazine and call your doctor immediately if you experience any of these serious symptoms:
With prolonged use, the risk of Drug-Induced Lupus increases. Patients on long-term therapy must have regular blood tests to check for Antinuclear Antibodies (ANA). Additionally, long-term use can lead to a deficiency in Vitamin B6, which may require supplementation to prevent nerve damage. Some patients may also develop a tolerance to the drug's effects, requiring dose adjustments or the addition of a diuretic to manage fluid retention.
No FDA black box warnings for Hydralazine. However, the warning regarding Drug-Induced Lupus is considered a 'Major Warning' in the prescribing information and is treated with similar clinical gravity. Healthcare providers are instructed to discontinue the drug immediately if a lupus-like syndrome develops, as the symptoms usually resolve once the medication is stopped, though it may take several months.
Report any unusual symptoms, especially persistent joint pain or fever, to your healthcare provider immediately.
Hydralazine is a high-potency antihypertensive that requires careful management. It is rarely used as a standalone therapy because the body's compensatory mechanisms (increased heart rate and fluid retention) can counteract its benefits. Therefore, it is almost always prescribed alongside a beta-blocker (to control heart rate) and a diuretic (to manage fluid buildup).
No FDA black box warnings for Hydralazine. While it lacks a boxed warning, the risk of drug-induced lupus is a primary clinical concern that dictates how the drug is monitored and prescribed.
Regular clinical monitoring is essential for anyone taking hydralazine:
Hydralazine can cause dizziness, lightheadedness, and blurred vision, especially during the first few weeks of treatment. Do not drive, operate heavy machinery, or engage in dangerous activities until you know how this medication affects you.
Alcohol should be avoided or strictly limited while taking hydralazine. Alcohol also dilates blood vessels and can cause an additive effect, leading to a dangerous drop in blood pressure, severe dizziness, and fainting.
Do not stop taking hydralazine abruptly. Doing so can lead to a sudden and severe increase in blood pressure (rebound hypertension), which increases the risk of stroke or heart attack. If the drug must be stopped, your doctor will provide a schedule to gradually taper the dose.
> Important: Discuss all your medical conditions, especially any history of heart disease, kidney issues, or stroke, with your healthcare provider before starting Hydralazine.
While there are few absolute 'never use' combinations, hydralazine should not be used with other potent vasodilators unless under expert supervision in a hospital setting.
For each major interaction, the primary concern is either an excessive drop in blood pressure (pharmacodynamic interaction) or a change in how the drug is metabolized in the liver. Management usually involves frequent blood pressure monitoring and dose adjustments.
> Important: Tell your doctor about ALL medications, supplements, and herbal products you are taking, including over-the-counter cold and pain medicines.
Hydralazine must NEVER be used in the following circumstances:
These conditions require a careful risk-benefit analysis by a physician:
There is no significant evidence of cross-sensitivity between hydralazine and other major classes of antihypertensives (like ACE inhibitors or sulfonamides). However, patients who have had severe reactions to other phthalazine derivatives should be treated with extreme caution.
> Important: Your healthcare provider will evaluate your complete medical history, including any history of heart valve disease or chest pain, before prescribing Hydralazine.
Hydralazine is classified as FDA Pregnancy Category C. This means that while animal studies have shown some risks, there are no adequate, well-controlled studies in humans. However, hydralazine has been used for decades in clinical practice to treat severe hypertension in pregnancy (preeclampsia and eclampsia). It is often considered a second-line or third-line choice after labetalol or nifedipine. It does cross the placenta, and its use in the first and second trimesters is generally avoided unless absolutely necessary. In the third trimester, it is primarily used in acute hospital settings to prevent maternal stroke.
Hydralazine is excreted into breast milk in very small amounts. According to the American Academy of Pediatrics, it is generally considered compatible with breastfeeding. However, because it can lower the mother's blood pressure significantly, the nursing infant should be monitored for any signs of unusual lethargy or poor feeding. Always consult your doctor before breastfeeding while on this medication.
Hydralazine is approved for use in children for the treatment of hypertension. However, safety and effectiveness data are more limited than in adults. It is typically reserved for children with resistant hypertension or those with underlying kidney disease. Pediatric dosing must be meticulously calculated based on weight (mg/kg) and adjusted frequently as the child grows.
Clinical studies have not identified significant differences in response between the elderly and younger patients. However, the elderly are at a much higher risk for 'orthostatic hypotension' (dizziness when standing) and falls. Age-related declines in kidney and liver function mean that hydralazine may stay in the system longer. Doctors typically 'start low and go slow' with dosing in patients over age 65.
In patients with a Glomerular Filtration Rate (GFR) less than 30 mL/min, the clearance of hydralazine is significantly reduced. This leads to a higher risk of side effects, including the lupus-like syndrome. Dosing intervals are often extended (e.g., every 8 to 12 hours instead of every 6 hours) to prevent accumulation.
Since the liver is the primary site of hydralazine metabolism (via N-acetylation), patients with cirrhosis or hepatitis may have much higher blood concentrations of the drug. These patients require close monitoring for signs of excessive vasodilation, such as severe flushing, headache, and low blood pressure.
> Important: Special populations, particularly pregnant women and the elderly, require individualized medical assessment and frequent follow-up appointments.
Hydralazine is a direct-acting peripheral vasodilator. Its primary molecular target is the vascular smooth muscle of the arterial system. While the exact receptor-binding site is still a subject of research, it is known to inhibit the release of calcium from the sarcoplasmic reticulum within smooth muscle cells. It also increases the local availability of nitric oxide, which stimulates guanylate cyclase. This leads to an increase in cyclic GMP, causing the muscle fibers to relax. This relaxation is selective for arterioles; it has almost no effect on the venous system (veins).
Following oral administration, the blood-pressure-lowering effect typically begins within 20 to 30 minutes, peaks at 1 to 2 hours, and lasts for 6 to 12 hours. When given intravenously, the effect begins within 5 to 20 minutes and peaks at 10 to 80 minutes. Because it causes a rapid drop in systemic vascular resistance, it frequently triggers a compensatory increase in heart rate (reflex tachycardia) and an increase in stroke volume.
| Parameter | Value |
|---|---|
| Bioavailability | 30% - 50% (Increases with food) |
| Protein Binding | 87% (Primarily Albumin) |
| Half-life | 2 - 8 hours (Longer in slow acetylators) |
| Tmax | 1 - 2 hours (Oral) |
| Metabolism | Hepatic (N-acetylation via NAT2) |
| Excretion | Renal (85%), Fecal (10%) |
Hydralazine hydrochloride is 1-hydrazinophthalazine monohydrochloride. Its molecular formula is C8H8N4•HCl, and it has a molecular weight of 196.64. It is a white to off-white, crystalline powder that is soluble in water. The structure is characterized by a phthalazine ring with a hydrazine substituent, which is essential for its vasodilatory properties.
Hydralazine is classified as a Peripheral Vasodilator. Within the Nitrate Vasodilator [EPC] framework, it is often grouped with medications like minoxidil (another direct vasodilator) and nitrates, though its specific mechanism of arterial-only dilation is distinct from the balanced arterial-venous dilation seen with organic nitrates.
Medications containing this ingredient
Common questions about Hydralazine
Hydralazine is primarily used to treat essential hypertension (high blood pressure) and heart failure. In hypertension, it is usually added when other medications have not worked well enough to reach target blood pressure levels. For heart failure, it is often used in combination with another drug called isosorbide dinitrate to help the heart pump more easily. It works by relaxing the blood vessels, which allows blood to flow more freely. Your doctor may also use the injectable form in emergency situations to lower dangerously high blood pressure quickly.
The most common side effects of Hydralazine include headaches, a fast or pounding heartbeat (palpitations), and nausea or vomiting. Many patients also experience dizziness, especially when standing up quickly, and flushing of the face. These symptoms are often most noticeable when you first start the medication or when your dose is increased. In most cases, these side effects become less severe as your body adjusts to the medicine over several weeks. If they persist or become bothersome, you should contact your healthcare provider for advice.
It is generally recommended that you avoid or strictly limit alcohol consumption while taking Hydralazine. Alcohol is also a vasodilator, meaning it widens blood vessels, and when combined with Hydralazine, it can cause your blood pressure to drop too low. This can lead to severe dizziness, lightheadedness, and an increased risk of fainting or falling. Alcohol can also increase the likelihood of experiencing a fast heartbeat or a severe headache. Always discuss your alcohol intake with your doctor to ensure your safety while on this medication.
Hydralazine has been used for many years to treat high blood pressure during pregnancy, particularly in emergency situations like preeclampsia. While it is classified as Category C, meaning there are some risks, it is often used when the benefits to the mother outweigh the potential risks to the baby. It is usually reserved for the later stages of pregnancy and is administered in a hospital setting. If you are pregnant or planning to become pregnant, your doctor will carefully evaluate whether Hydralazine is the safest option for you. Never start or stop this medication during pregnancy without direct medical supervision.
When taken by mouth, Hydralazine typically begins to lower blood pressure within 20 to 30 minutes. The maximum effect is usually reached about 1 to 2 hours after the dose is swallowed. The effects of a single dose generally last for about 6 to 12 hours, which is why the medication is often prescribed to be taken four times a day. If given by injection in a hospital, the drug works much faster, with effects starting in as little as 5 to 20 minutes. It may take several weeks of consistent use to see the full long-term benefits on your blood pressure.
No, you should never stop taking Hydralazine suddenly unless your doctor specifically tells you to do so. Stopping the medication abruptly can cause a 'rebound' effect, where your blood pressure rises very quickly to dangerous levels. This sudden spike increases the risk of serious complications like a heart attack or stroke. If you need to stop the medication, your doctor will usually provide a plan to gradually decrease your dose over several days or weeks. Always ensure you have enough medication and refill your prescription before you run out.
If you miss a dose of Hydralazine, take it as soon as you remember. However, if it is almost time for your next scheduled dose, skip the missed dose and continue with your regular dosing schedule. Do not take two doses at the same time to make up for the one you missed, as this can cause your blood pressure to drop to a dangerously low level. If you frequently forget to take your doses, consider using a pill organizer or setting an alarm on your phone. Consistency is key to keeping your blood pressure under control.
Hydralazine itself does not typically cause weight gain in the form of fat, but it can cause the body to retain salt and water, which leads to fluid weight gain. This fluid retention often presents as swelling in the ankles, feet, or lower legs (edema). If you notice a sudden, rapid increase in weight (such as 3 pounds in a day or 5 pounds in a week), you should contact your doctor immediately. This could be a sign that your body is holding too much fluid, and your doctor may need to adjust your medications or add a diuretic (water pill).
Hydralazine is frequently taken with other medications, but it can interact with many different drugs. It is most commonly prescribed alongside a beta-blocker and a diuretic to manage side effects like fast heart rate and fluid retention. However, it can interact with other blood pressure drugs, certain antidepressants, and even over-the-counter NSAIDs like ibuprofen. Because of these potential interactions, it is vital to provide your doctor and pharmacist with a complete list of all the medicines, vitamins, and herbal supplements you are currently taking.
Yes, Hydralazine is available as a generic medication and is generally very affordable. Generic hydralazine hydrochloride tablets are bioequivalent to the original brand-name versions, meaning they have the same active ingredient and work the same way in the body. Most insurance plans cover the generic version of this medication. Using the generic form is an effective way to manage high blood pressure while keeping healthcare costs lower. You should always check with your pharmacist to ensure you are receiving the correct formulation and strength prescribed by your doctor.