Hydralazine (Apresoline) | Medication Reference

Bedside Snapshot

Mechanism: Direct arteriolar vasodilator; drops systemic vascular resistance (afterload) with minimal effect on venous capacitance or AV node
Primary Uses: Severe hypertension in pregnancy (preeclampsia/eclampsia), selected hypertensive emergencies when titratable agents unavailable, chronic HFrEF with nitrates
Typical IV Dose (non-pregnant): 10–20 mg IV over ≥3–5 minutes; may repeat q4–6h, max ~40 mg/dose
Typical IV Dose (pregnancy): 5–10 mg IV over 2–3 minutes; repeat q20–30 min if BP ≥160/110 mmHg, total episode max 20–30 mg
Onset (IV): 5–20 minutes; peak effect ~10–30 minutes; duration 2–4 hours or longer (delayed and unpredictable)
Key Acute Risks: Excessive hypotension, reflex tachycardia, myocardial ischemia in CAD patients, precipitous BP drop in stroke
Key Chronic Risks: Drug-induced lupus (especially slow acetylators), peripheral neuropathy, fluid retention
Special Note: Not first-line for most hypertensive emergencies in modern critical care (nicardipine, clevidipine, labetalol preferred); remains common in obstetric protocols

Brand & Generic Names

Generic Name: Hydralazine
Brand Names: Apresoline (historical; now primarily available as generics; availability varies by institution and region)

Medication Class

Direct arteriolar vasodilator (afterload reducer); antihypertensive agent

Pharmacology

Mechanism of Action:

Direct arteriolar vasodilator; relaxes vascular smooth muscle in resistance vessels (arterioles) more than veins, producing decreased systemic vascular resistance and afterload
Proposed mechanisms include interference with intracellular calcium handling, opening of potassium channels, and increased nitric oxide availability
Net effect: decreased intracellular Ca²⁺ and smooth muscle relaxation
Reflex sympathetic activation occurs in response to BP drop: heart rate and cardiac output increase, renin–angiotensin–aldosterone activation promotes sodium and water retention
No direct depression of AV nodal conduction (tachycardia is reflex, not nodal)
Chronic exposure at higher doses can trigger drug-induced lupus, likely related to metabolite-driven immune alterations and genetic acetylation differences

Pharmacokinetics:

Absorption: IV administration 100% bioavailable; oral hydralazine rapidly absorbed but undergoes extensive first-pass metabolism with highly variable bioavailability (~10–55%), influenced by acetylator status and food
Onset (IV): BP reduction begins within ~5–20 minutes; peak effect ~10–30 minutes after bolus; effect persists 2–4 hours or longer
Onset (oral): Antihypertensive effect begins within 20–40 minutes; lasts 3–8 hours (requires dosing 2–4 times daily)
Distribution: Widely distributed; volume of distribution ~1–2 L/kg; ~85–90% protein bound
Metabolism: Primarily hepatic via N-acetyltransferase; acetylation rate genetically determined (slow vs rapid acetylators). Slow acetylators have higher parent-drug exposure and greater lupus risk; rapid acetylators may need higher doses
Elimination: Metabolites excreted renally; small fraction of unchanged drug in urine. Half-life variable, typically ~2–8 hours with normal hepatic/renal function
Renal impairment: Drug and metabolites accumulate with reduced GFR; increased risk of hypotension and toxicity; reduce dose and frequency, monitor closely
Hepatic impairment: Reduced metabolism increases exposure and prolongs effect; careful titration advised

Indications

Acute treatment of severe hypertension or hypertensive emergency/urgency when oral medications not feasible and titratable infusion not required or available (perioperative and ward settings)
Emergency treatment of severe hypertension in pregnancy (preeclampsia/eclampsia) when labetalol or nifedipine contraindicated, unavailable, or ineffective per obstetric guidelines
Adjunctive therapy in chronic hypertension when other agents insufficient, usually combined with beta-blockers and diuretics
Chronic management of heart failure with reduced ejection fraction (HFrEF) in combination with nitrates (e.g., isosorbide dinitrate), especially in patients unable to tolerate ACE inhibitors/ARBs/ARNIs (primarily oral indication)

Dosing & Administration

Available Forms:

IV injection: Hydralazine hydrochloride injection 20 mg/mL in 1 mL single-dose vials (20 mg total) for direct IV or IM use (dilution recommended for slow IV administration)
Oral tablets: Commonly 10 mg, 25 mg, 50 mg, and 100 mg strengths
Fixed-dose combination tablets for HFrEF: Hydralazine with isosorbide dinitrate (e.g., 20 mg/37.5 mg, 40 mg/75 mg)

Standard IV Dosing (Always Follow Local Protocols):

Indication / Population	Initial IV Dose	Repeat / Titration	Typical Maximum (Episode/24 h)
Adult severe hypertension / hypertensive emergency (non-pregnant)	10–20 mg IV over ≥3–5 min	May repeat q4–6 hr based on BP response; some refs allow 10–40 mg IV/IM not exceeding ~20 mg per bolus	Often limited to 40–50 mg in 24 h (institution-specific)
Adult pregnancy-associated severe hypertension (preeclampsia/eclampsia)	5–10 mg IV over 2–3 min	Repeat 5–10 mg IV q20–30 min if SBP ≥160 or DBP ≥110 mmHg; many protocols cap at 20–30 mg total per episode	Total 20–30 mg per treatment episode (then switch to alternative or continuous agent if needed)
Adult continuous IV infusion (where used; institution-specific)	0.5–10 mg/hr IV infusion after or instead of boluses	Titrate q20–30 min based on BP in monitored setting; rarely used in modern practice vs titratable CCB or beta-blocker infusions	Institution-specific; often not exceeding 20–40 mg over several hours
Pediatric severe hypertension (specialist use)	0.1–0.2 mg/kg IV (commonly 0.15 mg/kg); max initial 20 mg	May repeat per pediatric hypertension guideline (e.g., q4–6 hr) with close hemodynamic monitoring	Weight-based; usual total ≤0.4–0.6 mg/kg/day, not exceeding adult limits

Monitoring Note: Monitor BP and heart rate closely during and after IV administration; check BP every 5 minutes for at least 15–20 minutes after bolus in high-risk patients. Avoid stacking repeated doses too quickly—allow 20–30 minutes to assess full effect.

Oral Dosing (Chronic Therapy - Context Only):

Chronic hypertension: Commonly initiated at 10 mg PO four times daily for 2–4 days, then increased to 25 mg PO four times daily for a week, then to 50 mg PO four times daily as needed. Usual total daily dose 100–200 mg/day, up to 300 mg/day in resistant cases
For HFrEF (with nitrates): Fixed-dose combinations (e.g., hydralazine 20–40 mg with isosorbide dinitrate 37.5–75 mg) typically dosed three times daily and titrated per heart-failure guidelines
Important: Chronic hydralazine should almost never be used as monotherapy—pair with beta-blocker and diuretic to blunt reflex tachycardia and fluid retention

Contraindications

Absolute or Major Contraindications:

Known hypersensitivity to hydralazine or any component of the formulation
Severe coronary artery disease or acute coronary syndrome where reflex tachycardia and increased myocardial oxygen demand may precipitate ischemia or infarction (relative/strong caution)
Hypertrophic obstructive cardiomyopathy or significant fixed outflow obstruction (e.g., severe aortic stenosis), where sudden afterload reduction can worsen obstruction and hemodynamics
Marked tachycardia or high-output states where further sympathetic activation is undesirable
Known history of hydralazine-induced lupus or severe hydralazine-induced hepatotoxicity or blood dyscrasia

Major Precautions:

Heart failure with reduced EF: Acute afterload reduction may help in hypertensive emergencies but reflex tachycardia and fluid retention may worsen chronic HF if not paired with beta-blocker and diuretic
Cerebrovascular disease (e.g., acute ischemic stroke): Overly rapid BP reduction may worsen cerebral ischemia; disease-specific BP targets should guide dosing
Renal impairment: Accumulation of drug and metabolites increases risk of hypotension and toxicity; start low, go slow, monitor BP and renal function closely
Hepatic impairment and slow acetylator phenotype: Increased exposure and higher risk of lupus-like reactions and other adverse effects
Long-term therapy: Monitor for symptoms/signs of lupus-like syndrome (arthralgias, rash, fever), peripheral neuropathy, hepatotoxicity, and hematologic abnormalities; obtain baseline and periodic CBC, ANA, and liver enzymes per local protocols
Pregnancy: IV hydralazine widely used in pregnancy-associated severe hypertension but no longer considered superior to labetalol or nifedipine; selection should follow obstetric guidelines and available expertise

Coronary Disease Warning: In patients with known CAD, severe AS, or HOCM, hydralazine-induced reflex tachycardia and afterload shifts can precipitate ischemia or hemodynamic collapse. Alternative agents are usually preferred.

Adverse Effects

Common (especially with IV or higher oral doses):

Headache, flushing, and sensation of warmth
Palpitations and reflex sinus tachycardia
Dizziness, lightheadedness, orthostatic symptoms
Nausea, vomiting, diarrhea, or abdominal discomfort
Nasal congestion and edema/fluid retention with chronic use

Serious:

Symptomatic hypotension with syncope or shock, especially with rapid or repeated IV boluses
Myocardial ischemia or infarction precipitated by reflex tachycardia and increased oxygen demand in patients with coronary disease
Torsades de pointes or other arrhythmias in setting of excessive hypotension or coexisting QT-prolonging factors (rare but reported)
Drug-induced lupus-like syndrome (arthralgia, myalgia, fever, serositis, positive ANA), more common with higher chronic doses and in slow acetylators
Peripheral neuropathy (numbness, paresthesias), often associated with pyridoxine deficiency; may improve with vitamin B6 supplementation and dose reduction
Hepatotoxicity (elevated transaminases, rare fulminant liver failure) and hematologic effects (rare agranulocytosis, thrombocytopenia)

Drug-Induced Lupus Warning: Any new arthralgias, fevers, chest pain, rash, or serositis in a patient on chronic hydralazine should raise possibility of drug-induced lupus and prompt evaluation and consideration of discontinuation.

Special Populations

Pregnancy: Widely used IV for severe pregnancy-associated hypertension (preeclampsia/eclampsia); remains in many obstetric protocols. No longer considered superior to labetalol or nifedipine; agent selection should follow current obstetric guidelines
Lactation: Hydralazine is excreted in breast milk in small amounts; generally considered compatible with breastfeeding, but monitor infant for hypotension or other effects
Renal impairment: Both hydralazine and metabolites may accumulate; reduce initial dose and frequency, monitor BP and renal function closely. Increased risk of hypotension and toxicity
Hepatic impairment: Reduced metabolism can significantly increase exposure and prolong effect; use lower initial doses, titrate carefully, and avoid large IV boluses
Elderly: Increased sensitivity to hypotensive effects; start with lower doses and monitor closely. Higher risk of orthostatic hypotension
Slow acetylator phenotype: Genetically determined slower drug metabolism leads to higher drug levels, prolonged effect, and significantly increased risk of drug-induced lupus syndrome with chronic therapy

Monitoring

Clinical Monitoring (especially in ED/ICU and obstetric settings):

Noninvasive blood pressure every 5 minutes during and after IV bolus until stable; continuous arterial-line monitoring in critically ill patients is ideal
Heart rate and rhythm (telemetry) to detect reflex tachycardia, arrhythmias, or ischemic changes (ST depression, chest pain)
Clinical signs of organ perfusion: mental status, urine output, lactate, neurologic status in stroke, and fetal monitoring in pregnancy (per obstetric protocol)

Laboratory Monitoring:

Baseline and follow-up renal function (serum creatinine, urine output) and electrolytes, particularly when hydralazine combined with diuretics or other antihypertensives
For chronic oral therapy: Periodic CBC, liver enzymes, ANA (based on institutional practice), and assessment for lupus-like symptoms, neuropathy, edema, and adherence
In pregnancy: continuous fetal monitoring where indicated (e.g., severe preeclampsia with IV hydralazine), with documentation of maternal BP response and symptoms

Clinical Pearls

Slower Onset, Longer Duration: IV hydralazine has slower onset and more prolonged, variable effect than titratable IV infusions like nicardipine or clevidipine. Best for settings where you can tolerate delayed but sustained BP reduction.

Pregnancy Hypertension: Hydralazine remains in many obstetric protocols for severe pregnancy-associated hypertension but has largely been joined or supplanted by labetalol and nifedipine. Knowing all three regimens is valuable in obstetric emergencies.

Avoid Stacking Doses: Once you give 5–10 mg IV, wait long enough to see the full effect (often 20–30 minutes) unless there is compelling reason to re-dose sooner and monitoring is robust. Stacking doses can lead to severe hypotension.

BP Overshoot Management: If BP undershoots target or patient becomes symptomatic (dizziness, chest pain, altered mental status), be prepared to intervene with fluids, vasopressors, or other supportive measures, and reassess your BP goals.

Consider Patient Pathophysiology: In patients with coronary disease, severe AS, or HOCM, afterload reduction and reflex tachycardia may be dangerous. In patients with severe afterload-driven heart failure and intact coronaries, hydralazine may be helpful when used judiciously.

Chronic Therapy Triple Therapy: For chronic use, hydralazine is much cleaner when paired with a beta-blocker and a diuretic to blunt reflex sympathetic activation and fluid retention. As monotherapy it often generates as many problems (tachycardia, edema) as it solves.

Drug-Induced Lupus Surveillance: Any new arthralgias, fevers, chest pain, rash, or serositis in a patient on chronic hydralazine should raise the possibility of drug-induced lupus and prompt evaluation and consideration of discontinuation.

References

1. Herman, L. L., Padala, S. A., Ahmed, I., & Surani, S. (2023). Hydralazine. In StatPearls [Internet]. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK470296/
2. Alley, W. D., & colleagues. (2023). Hypertensive emergency. In StatPearls [Internet]. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK470371/
3. American Regent. (2013). Hydralazine hydrochloride injection, USP: Prescribing information. U.S. Food and Drug Administration. https://www.accessdata.fda.gov/drugsatfda_docs/label/2013/040136s005lbl.pdf
4. Medscape. (2025). Preeclampsia treatment & management. Medscape Reference. https://emedicine.medscape.com/article/1476919-treatment
5. Medscape. (2024). Apresoline (hydralazine) dosing, indications, interactions, adverse effects, and more. Medscape Reference. https://reference.medscape.com/drug/apresoline-hydralazine-342400