Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.

What is hydralazine?

Hydralazine (hai • draa • luh • zeen) is used to treat high blood pressure. It works by opening blood vessels and allowing blood to circulate freely.

Depending on what your doctor indicates, hydralazine may be used alone or with other medications to control your blood pressure. Hydralazine is a powerful drug that belongs to a class of drugs called vasodilators. In the US, it is available as a generic drug only. In some countries, hydralazine is marketed under the brand name Apresoline.

What is hydralazine used to treat?

Oral hydralazine is a drug used to treat high blood pressure.¹ Hydralazine is a type of vasodilator (a drug that opens blood vessels). 

Dilating (widening) the blood vessels allows for improved blood circulation.

How do you take hydralazine?

Hydralazine is usually taken orally. It is available in a 10mg, 25mg, 50mg, or 100mg tablet. Typically, hydralazine is taken between three to four times a day.²

Take hydralazine exactly as directed by your doctor. 

In a medical emergency, when healthcare professionals need to bring down dangerously high blood pressure quickly, they may give hydralazine intravenously (IV therapy).³

Hydralazine is used to treat high blood pressure but does not cure it.

Even if you feel well, you must follow your doctor’s instructions for taking hydralazine to maintain its therapeutic effects.

Do not stop taking hydralazine without consulting your doctor first.

Seeing results

Hydralazine is quick-acting.

Its blood pressure lowering effects can happen within 30 minutes of taking the medication orally or within 5–20 minutes when given by IV or injected into a muscle.⁴

Hydralazine’s effects will usually last between 2–6 hours, depending on how it is administered.

Longer-lasting effects happen with IV therapy or intramuscular injection.

Potential side effects of hydralazine

If you experience the following symptoms after taking hydralazine, speak with your doctor or seek medical attention.⁵

Common side effects

  • Headache

  • Anorexia

  • Nausea or vomiting

  • Diarrhea

  • Heart palpitations

  • Tachycardia (heart rate over 100 beats per minute)*

  • Angina pectoris (chest pain)*

*Call 911 or visit the nearest emergency room if you experience chest pain or rapid heart rate.

Less common side effects

  • Constipation

  • Temporary paralysis of the intestines (paralytic ileus)

  • Extremely low blood pressure (hypotension)

  • Raised blood pressure (paradoxical pressor response)

  • Fluid retention (edema)

  • Shortness of breath

  • Difficulty urinating

  • Blood dyscrasias (a diseased state of the blood)

  • Nasal congestion

  • Conjunctivitis

  • Flushing

  • Secretion of tears (lacrimation)

  • Rash

  • Hives

  • Itchy skin

  • Fever or chills

  • Joint pain

  • Lower than normal level of disease-fighting white blood cells.

  • Neurological symptoms: numbness, tingling, dizziness, tremors, muscle cramps, disorientation, anxiety, depressive psychosis

Rare side effects

  • Hydralazine-induced hepatitis

Long-term use of hydralazine

Hydralazine is not a first-line treatment for lowering blood pressure. It is generally prescribed when other blood pressure treatments have been ineffective or in a medical emergency, such as heart failure.¹ A long-term, high-dose course of hydralazine may cause drug-induced lupus erythematosus (DIL), a type of autoimmune disease.⁶ ⁷ Before prescribing hydralazine, your doctor will help you weigh the possible risks and benefits based on your health profile. 

Missed doses

If you miss a dose of hydralazine, take it as soon as possible. If it is almost time for your next dose, skip the missed dose and return to your regular dosing schedule. Do not take two doses of hydralazine at once.


While relatively rare, some of the symptoms associated with hydralazine overdose include:

  • Blood pressure that is too low (hypotension)

  • A heart rate that’s too fast (tachycardia)

  • Headache

  • Generalized skin flushing

  • Heart arrhythmia

  • Chest pain

  • Shock

  • Coma 

If you suspect or know that you’ve taken too much hydralazine, call 911 or seek immediate medical attention.

What to discuss with your doctor before taking hydralazine

Before taking hydralazine, be sure to talk to your doctor about:

  • Any medical conditions you have, especially: 

    • Coronary artery disease

    • Recent heart attack

    • Rheumatic heart disease of the mitral valve

    • Blood vessel disorders

    • Lupus

    • A history of prior strokes

    • Kidney problems

  • All the medications you are currently taking (prescribed or non-prescribed). 

  • All vitamins and supplements you are taking or planning to take.

  • Allergies to specific medications (especially hydralazine). 

  • Whether or not you consume alcohol, how much, and how frequently.

  • If you are pregnant or trying to get pregnant. 

  • If you are breastfeeding or planning to breastfeed. 

  • Suitable dietary and exercise regimens while using hydralazine to lower blood pressure.

Stopping hydralazine

Do not abruptly stop hydralazine.

 If you stop suddenly, your blood pressure may become uncontrolled, raising the risk of serious complications like heart disease, chest pain, or heart attack. If your doctor advises you to stop taking hydralazine, they will gradually taper your dosage to avoid the risk of any complications.

Hydralazine and pregnancy

Hydralazine is in US Food and Drug Administration (FDA) pregnancy Category C. Studies in animals show adverse effects of hydralazine on fetal development. There are no adequate and well-controlled studies on humans.⁸ Hydralazine should be avoided during the first two trimesters of pregnancy. It is not recommended during the third trimester. 

However, a doctor may prescribe hydralazine in the third trimester in situations where the potential benefits exceed the risks. Talk to your doctor if you are taking hydralazine and plan to breastfeed. While taking hydralazine it will be present in breast milk, though it is not known to have harmful effects on infants.⁸

Interactions with other drugs

Drugs that interact with hydralazine and cause unwanted side effects may include (but are not necessarily limited to):

Monoamine oxidase inhibitors (MAOIs)

Taking antidepressants like isocarboxazid (Marplan), phenelzine (Nardil), or tranylcypromine (Parnate) while taking hydralazine can be dangerous and require medical attention. Ask your doctor about suitable medication for managing blood pressure levels while taking antidepressants.⁹


Taking aspirin while on hydralazine may increase the risk of getting dizzy, having headaches, or blurry vision. Ask your doctor for advice. They may need to adjust your dosage of hydralazine or monitor you more closely.

Cholesterol-lowering medications

Statins that lower cholesterol like rosuvastatin (Crestor¹²) or atorvastatin (Lipitor¹¹) may be unsuitable while taking hydralazine. Taking cholesterol drugs while on hydralazine could increase the risk of side effects from both drugs, including nerve damage.¹¹ Discuss the situation with your doctor.

Allergy information

Hydralazine may cause a severe allergic reaction. Symptoms of a severe reaction may include hives, difficulty breathing, and swelling of the face, lips, tongue, or throat. If you develop these symptoms upon taking hydralazine, seek immediate medical attention. Call 911 or go to the nearest emergency room. Older adults may be more likely to have adverse side effects from this medication.¹³

Clinical trial history

Hydralazine is not a first-line treatment for high blood pressure but is indicated for use when blood pressure isn’t sufficiently controlled by antihypertensive medication.¹⁴ Hydralazine was first developed to treat malaria but was later discovered to have blood-pressure-lowering properties. Hydralazine was approved by the US Food and Drug Administration (FDA) in 1953.¹⁵

Tips and advice for taking hydralazine

Here are some tips and advice for when you are taking hydralazine:

  • Take hydralazine with or without food. 

  • Taking hydralazine with food improves absorption.

  • Take hydralazine at the same time(s) every day.

  • Hydralazine can cause dizziness.

  • Avoid driving, using machinery, or doing activities that require full attention. 

  • Take your time when going from sitting or lying down to standing (your blood pressure may drop if you rise too quickly and make you dizzy).

  • Do not consume alcohol while taking hydralazine.

  • Alcohol, dehydration, exercise, or illness may increase the risk of dizziness.

  • Tell your doctor if you have any chest discomfort or heart palpitations after taking hydralazine.

  • If you have a rash, a fever, joint or muscular discomfort, breathing difficulties, numbness, or nerve pain in your hands or feet, seek immediate medical attention.

  • Before using over-the-counter pain relievers or other over-the-counter medications, speak with your doctor or pharmacist to ensure that they are safe to use while taking hydralazine.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.

Explore related clinical trials

Actively recruiting
Heart Health Study
Actively recruiting
Assessing the Efficacy and Safety of an Investigational Drug in Individuals with Uncontrolled Hypertension
Actively recruiting
Effects of Sodium-glucose Co-transporter-2( SGLT-2 ) Inhibition on Sympathetic Nervous System Activity in Humans
View related condition trials page

Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.