Pharmacology · Direct vasodilators
Hydralazine !
A vasodilator that relaxes the arterioles. It is used intravenously in severe hypertension of pregnancy and, with a nitrate, in heart failure.
Updated 2026-11-01
Mechanism of action
Hydralazine relaxes arteriolar smooth muscle directly, with little effect on veins; the precise molecular mechanism is not fully understood. Diastolic pressure falls more than systolic, peripheral resistance falls, and the baroreflex raises heart rate, stroke volume and cardiac output. Renin release causes salt and water retention.
- Hydralazine relaxes arteriolar smooth muscle
- Peripheral resistance and diastolic pressure fall
- Baroreflex: heart rate and cardiac output rise
- Renin rises: salt and water are retained
Pharmacokinetics
- Absorption
- Rapidly absorbed by mouth; marked first-pass metabolism
- Metabolism
- Acetylation by NAT2: slow acetylators have higher levels
- Half-life
- About 3 to 7 hours (label); the effect outlasts plasma levels
Indications
- High blood pressure Arterial hypertension →
- Severe hypertension in pregnancy, including preeclampsia (intravenous) Preeclampsia →
- Heart failure with reduced ejection fraction, combined with isosorbide dinitrate Heart failure →
Adverse effects
Common
- Headache
- Palpitations and tachycardia
- Nausea, vomiting, diarrhoea and anorexia
- Fluid retention and oedema
Contraindications
- Coronary artery disease
- Mitral valve disease of rheumatic origin
- Hypersensitivity to hydralazine
Key interactions
- Diazoxide and other potent parenteral antihypertensives
- Profound hypotension
- MAO inhibitors
- Used with caution: stronger hypotension
- Beta blockers and diuretics
- Used together on purpose to block reflex tachycardia and fluid retention
What is monitored
- Blood pressure and heart rate
- Complete blood count and antinuclear antibodies during long treatment
- Symptoms of lupus: joint pain, fever, rash
- Oedema and weight
Nursing care
- During intravenous use in pregnancy, blood pressure is checked often and the fetal heart rate is monitored.
- Report chest pain, palpitations or fast heartbeat.
- Report joint pain, fever, chest pain or a rash, which can signal a lupus-like reaction.
- Numbness or tingling can reflect pyridoxine deficiency; tell the prescriber.
Pregnancy and breastfeeding
There are no adequate controlled studies, but clinical experience has not shown harm to the fetus, and intravenous hydralazine is a standard option for severe hypertension in pregnancy.
Test yourself
Think of the answer, then tap the question to see it.
Why does hydralazine cause tachycardia and fluid retention?
The fall in arteriolar resistance activates the baroreflex (tachycardia) and renin release (salt and water retention).
Which immune adverse effect is typical of hydralazine, and who is at higher risk?
A drug-induced lupus-like syndrome; it is linked to slow acetylation by NAT2, which raises drug levels.
Why is hydralazine contraindicated in coronary artery disease?
Reflex tachycardia and higher cardiac output increase myocardial oxygen demand and can trigger angina or ischaemia.
With which drug is hydralazine combined in heart failure?
Isosorbide dinitrate: hydralazine lowers afterload and the nitrate lowers preload.
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - HYDRALAZINE HYDROCHLORIDE tablet (DailyMed (U.S. National Library of Medicine), 2026); Hydralazine: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2017); eEML - Electronic Essential Medicines List (World Health Organization, 2025); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 32 (McGraw Hill, 2023).
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