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Hydralazine (Apresoline) vs. hydrochlorothiazide (Microzide)

Hydralazine vs. hydrochlorothiazide: What’s the difference?

What is Hydralazine? What is Hydrochlorothiazide?

Hydralazine is a peripheral arterial vasodilator used as an antihypertensive medication to treat high blood pressure, hypertensive crisis, and congestive heart failure (CHF). Hydralazine causes relaxation of blood vessels which carry blood away from the heart and towards the organs and tissues. Hydrazine affects calcium movement within blood vessels. Calcium is required for muscle contraction and therefore disturbances in calcium movement may cause smooth muscle relaxation in the blood vessels. Hydralazine is selective for arterioles (small arteries), and the overall effects of treatment include a decrease in arterial blood pressure, and peripheral vascular resistance.

Hydrochlorothiazide is a diuretic (water pill) used to treat high blood pressure (hypertension). Hydrochlorothiazide is used to treat excessive fluid accumulation and swelling (edema) of the body caused by heart failure, cirrhosis, chronic kidney failure, corticosteroid medications, and nephrotic syndrome. It works by blocking salt and fluid reabsorption from the urine in the kidneys, causing increased urine output (diuresis). Hydrochlorothiazide can be used to treat calcium-containing kidney stones because it decreases the amount of calcium excreted by the kidneys in the urine and thus decreases the amount of calcium in urine to form stones.


Salt and sodium are the same.
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What are the side effects of hydralazine and hydrochlorothiazide?


Common side effects of hydralazine are:

Other less commonly reported side effects include:

  • constipation,
  • paralytic ileus,
  • low blood pressure,
  • edema,
  • paradoxical pressor response,
  • dyspnea (shortness of breath),
  • peripheral neuritis,
  • numbness,
  • tingling,
  • tremors,
  • muscle cramps,
  • psychotic reactions,
  • difficulty urinating,
  • blood disorders,
  • hypersensitivity type reactions,
  • nasal congestion,
  • flushing,
  • abnormal tear production (lacrimation) , and
  • conjunctivitis.


Side effects of hydrochlorothiazide include

Patients allergic to sulfa may also be allergic to hydrochlorothiazide because of the similarity in the chemical structure of the medications.

Hydrochlorothiazide can aggravate kidney dysfunction and is used with caution in patients with kidney disease. Hydrochlorothiazide can lower blood potassium, sodium, and magnesium levels. Low potassium and magnesium levels can lead to abnormalities in heart rhythm, especially in patients already taking digoxin (Lanoxin). During hydrochlorothiazide treatment, supplementation with potassium is common to prevent low potassium levels.

Blood uric acid levels can increase during hydrochlorothiazide treatment, and this elevation may cause an episode of acute gout in some individuals. Thiazide diuretics may increase blood sugar (glucose) levels and precipitate diabetes.

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What is the dosage for hydralazine and hydrochlorothiazide?


Hypertension in adult patients:

  • Start treatment at a low dose and increase gradually based on individual patient response.
  • The usual recommended starting regimen is 10 mg orally four times daily for the first 2 to 4 days, increase to 25 mg four times daily for the remainder of the first week.
  • For the second and subsequent weeks, increase dosage to 50 mg four times daily (increase dose by 10-25 mg/dose gradually every 2-5 days).
  • The maximum daily dose used in clinical studies is 300 mg.

Congestive heart failure:

  • The initial dose for treating heart failure is 10 to 25 mg orally 3 to 4 times daily.
  • The usual dose is 225 to 300 mg daily in 3 to 4 divided doses.
  • For pediatric hypertensive emergencies, the dose is 0.5 mg/Kg IV every 4 hours.


  • Hydrochlorothiazide may be taken with or without food.
  • The usual adult dose for hypertension is 12.5 to 50 mg once daily.
  • The usual adult dose for treating edema is 25-100 mg once daily or in divided doses.

What drugs interact with hydralazine and hydrochlorothiazide?


The following sympathomimetics may decrease the effectiveness of hydralazine:

Co-administration of these agents may block the anti-hypertensive benefits of hydralazine treatment.


Hydrochlorothiazide reduces the elimination of lithium (Lithobid, Eskalith) by the kidneys and can lead to lithium toxicity.

Nonsteroidal anti-inflammatory drugs (NSAIDs), for example, ibuprofen (Motrin), may reduce the blood pressure lowering effects of hydrochlorothiazide. Blood sugar levels can be elevated by hydrochlorothiazide, necessitating adjustment in the doses of medications that are used for treating diabetes.

Combining hydrochlorothiazide with corticosteroids may increase the risk for low levels of blood potassium and other electrolytes. Low blood potassium (hypokalemia) can increase the toxicity of digoxin (Lanoxin).

Cholestyramine (Questran, Questran Light) and colestipol (Colestid) bind to hydrochlorothiazide and reduce its absorption from the gastrointestinal tract by 43% to 85%.

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Are hydralazine and hydrochlorothiazide safe to use when pregnant or breastfeeding?


Hydralazine should be used in pregnancy only if the potential benefit justifies the potential risk to the fetus. Hydralazine is classified as FDA pregnancy risk category C (Animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks). It is used for treating high blood pressure during pregnancy.

Hydralazine is excreted in breast milk. Hydralazine should be used cautiously in females who are breastfeeding if treatment is necessary.


There are no adequate studies of hydrochlorothiazide in pregnant women. Thiazides may increase the risk of fetal or neonatal jaundice, low platelet levels, and possibly other adverse reactions that have occurred in adults.

Hydrochlorothiazide is excreted in breast milk. Intense diuresis using hydrochlorothiazide may reduce the production of breast milk. Otherwise hydrochlorothiazide is considered safe to use during nursing if required by the mother.

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