A blood vessel relaxer that lowers blood pressure by acting directly on the muscle in artery walls. Tablets treat essential high blood pressure, and the injection is for severe high blood pressure when pills cannot be taken or pressure must come down quickly.
Labeled use. Essential hypertension (tablets); severe essential hypertension needing urgent or non-oral treatment (injection).
IV hydralazine is a long-standing option for acute severe hypertension in pregnancy and preeclampsia, alongside IV labetalol and oral nifedipine; pregnancy is not a separately labeled indication. By mouth it is rarely used alone for primary hypertension, because reflex tachycardia and fluid retention follow, so it is usually paired with a beta blocker and a diuretic. Combined with isosorbide dinitrate it treats heart failure with reduced ejection fraction, a product outside this record. Avoid it in coronary artery disease, where reflex tachycardia can provoke angina.
A direct-acting peripheral vasodilator; the precise mechanism is not fully understood. By altering calcium movement in vascular smooth muscle, it relaxes arterioles more than veins, lowering peripheral resistance and diastolic more than systolic pressure. Reflex sympathetic discharge raises heart rate, stroke volume, cardiac output, and plasma renin, so angiotensin II and aldosterone drive sodium retention. Renal and cerebral blood flow are maintained or increased. Oral peak levels come at 1 to 2 hours with a half-life of 3 to 7 hours; IV doses give their average maximal fall at 10 to 80 minutes.
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The labels do not describe a narrow therapeutic index or direct dosing by measured blood levels of the drug.
Doses come from the FDA label. Study tool, not for clinical decisions.
No dose table. Patients with marked renal damage may need a lower injection dose, and the label advises caution in advanced renal damage. In hypertensive patients with normal kidneys, renal blood flow rises and glomerular filtration is maintained.
No labeled dose change. Hydralazine undergoes extensive hepatic metabolism, including polymorphic acetylation; slow acetylators generally have higher levels and need lower doses.
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No specific antidote. Overdose causes hypotension, tachycardia, headache, and flushing and can lead to myocardial ischemia, arrhythmias, and profound shock. Supporting the circulation comes first: treat shock with plasma expanders, avoid vasopressors if possible, and treat tachycardia with a beta blocker. After an oral overdose, empty the stomach with airway protection and consider activated charcoal once the circulation is stable.
Pregnancy Category C. Teratogenic in mice, and possibly rabbits, at many times the human dose (cleft palate and facial and skull bone malformations), but not in rats. There are no adequate human studies, and clinical experience has shown no clear fetal harm; use only if the expected benefit justifies the risk to the fetus.
The tablet label states that hydralazine passes into breast milk. The injection label advises caution when the injection is given to a nursing woman.
Not established in controlled trials, though there is pediatric experience. Oral: 0.75 mg/kg per day in 4 divided doses, increased gradually over 3 to 4 weeks to at most 7.5 mg/kg or 200 mg daily. IM or IV: 1.7 to 3.5 mg/kg per day, divided into 4 to 6 doses.