Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
HYDRALAZINE HYDROCHLORIDE vs Hydralazine
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Vasodilation of arterioles by direct relaxation of vascular smooth muscle, likely involving interference with calcium movement.
Hydralazine is a direct-acting vasodilator that relaxes arteriolar smooth muscle, leading to decreased peripheral vascular resistance and blood pressure. Its exact mechanism is unclear but may involve interference with calcium movement and increased c GMP levels.
Hypertension,Heart failure (adjunctive therapy)
Hypertension (adjunctive therapy),Off-label: Chronic heart failure (in combination with isosorbide dinitrate, especially in African American patients)
Oral: Initiate with 10 mg 4 times daily for 2-4 days, then increase to 25 mg 4 times daily for the remainder of the week, then titrate to 50 mg 4 times daily. Maximum daily dose: 300 mg. Intravenous: 5-20 mg IV bolus, may repeat every 20-30 minutes as needed, or continuous IV infusion 0.5-10 mg/hour.
10-50 mg orally every 6 hours, titrate to maximum 300 mg/day; 10-20 mg intramuscularly or intravenously every 4-6 hours as needed.
The terminal elimination half-life of hydralazine is approximately 2–4 hours in patients with normal renal function, but it is prolonged in renal impairment (up to 7–16 hours). The antihypertensive effect often lasts longer than the half-life due to persistent binding to arteriolar receptors.
The terminal elimination half-life of hydralazine is approximately 2-4 hours in patients with normal renal function. However, the duration of antihypertensive effect may be longer (6-12 hours) due to tissue binding and slow release from vascular smooth muscle. In renal impairment, half-life may extend to 7-16 hours, necessitating dose adjustment.
Extensively metabolized in the liver via N-acetylation (N-acetyltransferase 2, NAT2) and subsequent conjugation; also metabolized by cytochrome P450 (CYP) enzymes.
Primarily hepatic via acetylation (N-acetyltransferase 2, NAT2). Metabolites include hydralazine pyruvic acid hydrazone and acetylhydralazine.
Hydralazine is primarily metabolized in the liver via N-acetylation (polymorphic) and hydroxylation. Less than 10% of the dose is excreted unchanged in urine. The major metabolites are hydralazine pyruvic acid hydrazone and other conjugates, which are excreted renally. Fecal elimination is negligible.
Hydralazine is primarily metabolized in the liver via N-acetylation and hydroxylation. Approximately 80-90% of the drug is eliminated in urine as metabolites, with less than 10% excreted unchanged. A small fraction appears in feces via biliary excretion.
Approximately 85–90% bound to plasma proteins, primarily albumin and alpha-1-acid glycoprotein.
85-90% bound primarily to albumin and alpha-1-acid glycoprotein.
1.5–1.8 L/kg. This large Vd indicates extensive distribution into tissues, including arteriolar smooth muscle.
1.5-2.0 L/kg. This high Vd indicates extensive tissue binding and accumulation in vascular smooth muscle.
Oral bioavailability is about 26–50% due to significant first-pass metabolism. Bioavailability is higher in slow acetylators compared to rapid acetylators.
Oral: 10-30% due to extensive first-pass metabolism. Bioavailability is lower in fast acetylators. Bioavailability is 100% for intravenous administration.
Cr Cl 10-50 m L/min: Administer every 8 hours. Cr Cl <10 m L/min: Administer every 8-16 hours. Dose reduction may be necessary to avoid accumulation.
GFR 10-50 m L/min: administer every 8-12 hours; GFR <10 m L/min: administer every 12-24 hours.
Child-Pugh Class A and B: No specific recommendations; use with caution. Child-Pugh Class C: Contraindicated due to risk of hepatotoxicity and reduced drug clearance.
Child-Pugh A: caution, consider starting at 25% of normal dose; Child-Pugh B: reduce dose by 50%; Child-Pugh C: contraindicated.
Oral: 0.75-1 mg/kg/day divided every 6-12 hours, maximum 5 mg/kg/day. Intravenous: 0.1-0.2 mg/kg/dose IV every 4-6 hours as needed, maximum 0.5 mg/kg/dose (20 mg).
0.75-1 mg/kg/day orally divided every 6-12 hours, maximum 7.5 mg/kg/day; intravenous: 0.1-0.2 mg/kg/dose every 4-6 hours as needed.
Initiate at lower doses (e.g., 10 mg 2-3 times daily) and titrate slowly due to increased risk of hypotension and drug accumulation; monitor renal function closely.
Start at 10 mg orally twice daily, increase slowly; monitor for hypotension and reflex tachycardia; maximal dose 200 mg/day.
May cause a syndrome resembling systemic lupus erythematosus (SLE), especially with prolonged use or high doses.
Systemic lupus erythematosus-like syndrome (drug-induced lupus) with long-term use at high doses.
May cause drug-induced lupus, peripheral neuritis (pyridoxine deficiency), myocardial infarction (precipitate angina), hypotension, tachycardia, and blood dyscrasias. Use with caution in patients with coronary artery disease, cerebrovascular disease, or renal impairment.
May cause drug-induced lupus erythematosus; discontinue if symptoms develop.,Peripheral neuritis (pyridoxine deficiency) with long-term use.,May precipitate angina or myocardial infarction in patients with coronary artery disease.,Tachycardia and palpitations may occur; use with beta-blockers if needed.,Blood dyscrasias (rare).
Hypersensitivity to hydralazine, mitral valvular rheumatic heart disease, coronary artery disease, and idiopathic systemic lupus erythematosus.
Hypersensitivity to hydralazine,Coronary artery disease (angina pectoris, myocardial infarction),Mitral valve rheumatic heart disease,Aortic aneurysm (relative)
Hydralazine absorption is significantly increased when taken with food; it is recommended to take with meals for consistent effect. Avoid high-tyramine foods (aged cheeses, cured meats, fermented products) if combined with MAOIs, though hydralazine itself is not an MAOI. No specific dietary restrictions otherwise.
Take with food or milk to reduce stomach upset and slow absorption. Avoid high-tyramine foods (e.g., aged cheese, cured meats, fermented products) if taking a combination product containing hydralazine and hydrochlorothiazide–though hydralazine alone has no known significant food interactions. Limiting salt intake enhances antihypertensive effect.
First trimester: Limited human data; animal studies show no teratogenicity. Second/third trimester: Associated with maternal hypotension and potential fetal distress; no known structural anomalies.
FDA Pregnancy Category C. First trimester: No well-controlled studies; animal studies show no evidence of teratogenicity but embryotoxicity at high doses. Second/Third trimesters: Associated with maternal hypotension potentially reducing placental perfusion; no fetal malformations reported, but neonatal thrombocytopenia, lupus-like syndrome, and arrhythmias reported with chronic use near term.
Hydralazine is excreted into breast milk in small amounts (M/P ratio ~0.8). Considered compatible with breastfeeding by AAP; monitor infant for hypotension or drowsiness.
Excreted into breast milk in small amounts (M/P ratio estimated 1.0-1.4). No reported adverse effects in infants. American Academy of Pediatrics considers compatible with breastfeeding. Monitor infant for hypotensive effects or drug accumulation, especially in neonates or preterm infants.
Increased clearance in pregnancy may require higher doses to achieve same antihypertensive effect; start low and titrate based on blood pressure response.
Increased volume of distribution and plasma clearance may require dose adjustments. Initial oral: 10 mg 4 times daily, titrate up to 300 mg/day. In severe hypertension, IV bolus (5-10 mg) may be used with caution. Monitor for reflex tachycardia and hypotension; dose titration based on maternal response.
Hydralazine is a direct-acting arterial vasodilator; its antihypertensive effect is limited by reflex tachycardia and fluid retention, so it is typically used in combination with a beta-blocker and a diuretic. Slow acetylators are at increased risk of drug-induced lupus, especially with doses >200 mg/day. Administer with food to enhance bioavailability; onset of action occurs within 20-30 minutes IV. For hypertensive urgency, IV hydralazine 5-20 mg every 20-30 min is used, but avoid in suspected myocardial ischemia due to reflex tachycardia.
Hydralazine is a direct-acting vasodilator; first dose may cause profound hypotension—administer with food to减缓 absorption. Monitor for drug-induced lupus (especially in slow acetylators); baseline ANA recommended. Tachyphylaxis occurs; combine with beta-blocker and diuretic to counteract reflex tachycardia and fluid retention. Adjust dose in renal impairment (creatinine clearance < 10 m L/min).
Take exactly as prescribed, with food or milk to increase absorption.,Do not stop abruptly; sudden cessation can cause severe rebound hypertension.,Report symptoms like chest pain, rapid heartbeat, joint pain, rash, or fever to your doctor.,Avoid alcohol and other antihypertensives unless approved by your doctor.,Inform your doctor if you become pregnant or plan to become pregnant.,May cause dizziness; rise slowly from sitting or lying down.
Take this medication exactly as prescribed, usually 3-4 times daily. Swallow tablets whole with a glass of water.,Do not stop taking this drug suddenly, as this may cause a rapid increase in blood pressure.,This medication may cause dizziness or lightheadedness, especially when getting up from a sitting or lying position. Rise slowly.,Report any unexplained fever, joint pain, rash, or sore throat to your healthcare provider immediately—these could be signs of a lupus-like reaction.,Avoid alcohol, as it can increase side effects like dizziness or drowsiness.,If you miss a dose, take it as soon as you remember unless it is almost time for the next dose. Do not double the dose.
"Oxaprozin, a nonsteroidal anti-inflammatory drug (NSAID), can reduce the antihypertensive efficacy of hydralazine, a direct-acting vasodilator. NSAIDs inhibit prostaglandin synthesis, which can lead to sodium and fluid retention and increased vascular resistance, thereby counteracting the vasodilatory effects of hydralazine. This interaction may result in diminished blood pressure control and require dosage adjustments or alternative therapies."
"Hydralazine, a direct-acting vasodilator, may reduce the antihypertensive efficacy of nonsteroidal anti-inflammatory drugs (NSAIDs) like sulindac. NSAIDs inhibit cyclooxygenase-mediated prostaglandin synthesis, leading to sodium retention and increased vascular tone, which can antagonize the vasodilatory effects of hydralazine. This interaction may result in elevated blood pressure and diminished control of hypertension in patients receiving both agents."
"Hydralazine, a direct-acting vasodilator, may reduce the antihypertensive efficacy of Tolfenamic acid, a nonsteroidal anti-inflammatory drug (NSAID) that non-selectively inhibits cyclooxygenase (COX) enzymes. The interaction arises because Tolfenamic acid's inhibition of COX-2 reduces synthesis of vasodilatory prostaglandins (e.g., prostacyclin) in the vascular endothelium, which counteracts the vasodilation induced by Hydralazine. Clinically, this can lead to blunted blood pressure reduction, potentially requiring dose adjustments or alternative therapies to maintain adequate hypertension control."
"Oxaprozin, a nonsteroidal anti-inflammatory drug (NSAID), can reduce the antihypertensive efficacy of hydralazine, a direct-acting vasodilator. NSAIDs inhibit prostaglandin synthesis, which can lead to sodium and fluid retention and increased vascular resistance, thereby counteracting the vasodilatory effects of hydralazine. This interaction may result in diminished blood pressure control and require dosage adjustments or alternative therapies."
"Hydralazine, a direct-acting vasodilator, may reduce the antihypertensive efficacy of nonsteroidal anti-inflammatory drugs (NSAIDs) like sulindac. NSAIDs inhibit cyclooxygenase-mediated prostaglandin synthesis, leading to sodium retention and increased vascular tone, which can antagonize the vasodilatory effects of hydralazine. This interaction may result in elevated blood pressure and diminished control of hypertension in patients receiving both agents."
"Hydralazine, a direct-acting vasodilator, may reduce the antihypertensive efficacy of Tolfenamic acid, a nonsteroidal anti-inflammatory drug (NSAID) that non-selectively inhibits cyclooxygenase (COX) enzymes. The interaction arises because Tolfenamic acid's inhibition of COX-2 reduces synthesis of vasodilatory prostaglandins (e.g., prostacyclin) in the vascular endothelium, which counteracts the vasodilation induced by Hydralazine. Clinically, this can lead to blunted blood pressure reduction, potentially requiring dose adjustments or alternative therapies to maintain adequate hypertension control."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about HYDRALAZINE HYDROCHLORIDE vs Hydralazine, answered by our medical review team.
HYDRALAZINE HYDROCHLORIDE is a Vasodilator that works by Vasodilation of arterioles by direct relaxation of vascular smooth muscle, likely involving interference with calcium movement.. Hydralazine is a Vasodilator that works by Hydralazine is a direct-acting vasodilator that relaxes arteriolar smooth muscle, leading to decreased peripheral vascular resistance and blood pressure. Its exact mechanism is unclear but may involve interference with calcium movement and increased c GMP levels.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between HYDRALAZINE HYDROCHLORIDE and Hydralazine depend on the specific clinical indication. These are both Vasodilator agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of HYDRALAZINE HYDROCHLORIDE is: Oral: Initiate with 10 mg 4 times daily for 2-4 days, then increase to 25 mg 4 times daily for the remainder of the week, then titrate to 50 mg 4 times daily. Maximum daily dose: 300 mg. Intravenous: 5-20 mg IV bolus, may repeat every 20-30 minutes as needed, or continuous IV infusion 0.5-10 mg/hour.. The standard adult dose of Hydralazine is: 10-50 mg orally every 6 hours, titrate to maximum 300 mg/day; 10-20 mg intramuscularly or intravenously every 4-6 hours as needed.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
A moderate-severity drug interaction has been identified when combining HYDRALAZINE HYDROCHLORIDE and Hydralazine. Oxaprozin, a nonsteroidal anti-inflammatory drug (NSAID), can reduce the antihypertensive efficacy of hydralazine, a direct-acting vasodilator. NSAIDs inhibit prostaglandin synthesis, which can lead to sodium and fluid retention and increased vascular resistance, thereby counteracting the vasodilatory effects of hydralazine. This interaction may result in diminished blood pressure control and require dosage adjustments or alternative therapies. Consult your prescriber before combining these medications.
The maternal-fetal safety profiles differ. HYDRALAZINE HYDROCHLORIDE is classified as Category A/B. First trimester: Limited human data; animal studies show no teratogenicity. Second/third trimester: Associated with maternal hypotension and potential fetal distress; no known stru. Hydralazine is classified as Category A/B. FDA Pregnancy Category C. First trimester: No well-controlled studies; animal studies show no evidence of teratogenicity but embryotoxicity at high doses. Second/Third trimesters: . Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.