Is hydralazine an antihistamine?

Hydralazine is Not an Antihistamine

Hydralazine is a direct-acting vasodilator used primarily for hypertension management, not an antihistamine. 1

Pharmacological Classification and Mechanism

Hydralazine belongs to the class of direct vasodilators, as clearly categorized in multiple ACC/AHA guidelines. It works by:

• Directly relaxing vascular smooth muscle, particularly in arterioles
• Causing vasodilation, which reduces peripheral vascular resistance
• Decreasing blood pressure through this vasodilatory effect

The medication has no antihistamine properties or effects on histamine receptors. According to the ACC/AHA guidelines, hydralazine is specifically classified under "Direct vasodilators" rather than antihistamines or histamine receptor antagonists. 1

Clinical Uses of Hydralazine

Hydralazine is primarily used for:

1. Hypertensive emergencies: Used intravenously with an initial dose of 10 mg via slow IV infusion, repeated every 4-6 hours as needed 1

2. Chronic hypertension management: Used orally at doses of 100-200 mg daily, divided into 2-3 doses 1

3. Heart failure therapy: Often combined with isosorbide dinitrate, particularly beneficial in African American patients 1, 2

Side Effects and Considerations

Hydralazine's side effect profile is consistent with its vasodilator classification, not with antihistamine properties:

• Reflex tachycardia (due to vasodilation)
• Sodium and water retention
• Headache and gastrointestinal complaints (especially when combined with isosorbide dinitrate) 2
• At higher doses, risk of drug-induced lupus-like syndrome 1, 3
• Unpredictable response and prolonged duration of action 1

Important Clinical Distinctions

The ACC/AHA guidelines specifically classify hydralazine as a direct vasodilator, which is fundamentally different from antihistamines in:

• Mechanism of action (vascular smooth muscle relaxation vs. histamine receptor blockade)
• Clinical indications (hypertension/heart failure vs. allergic conditions)
• Side effect profile (vasodilation-related effects vs. sedation and anticholinergic effects)

Practical Implications

Understanding that hydralazine is not an antihistamine is crucial because:

• Misclassifying medications can lead to inappropriate therapeutic expectations
• Hydralazine would not be effective for allergic conditions where antihistamines are indicated
• The monitoring parameters for hydralazine therapy focus on blood pressure and heart rate, not allergy symptoms

In summary, hydralazine is definitively classified as a direct vasodilator used for blood pressure management and heart failure, with no antihistamine properties or effects on histamine receptors.