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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Direct vasodilators

Medicines that relax the muscle of blood vessels directly. They lower blood pressure strongly but trigger a fast heart rate and fluid retention.

Updated 2026-11-01

## What they are

Drugs that act on vascular smooth muscle itself rather than on hormones or nerves: hydralazine and minoxidil dilate arterioles; sodium nitroprusside, an intravenous nitric oxide donor, dilates arteries and veins and is used in hypertensive emergencies.

## Mechanism of action

Hydralazine relaxes arteriolar muscle by a mechanism not fully understood; minoxidil, through minoxidil sulfate, opens ATP-sensitive potassium channels; nitroprusside releases nitric oxide, which activates guanylyl cyclase and raises cGMP. The fall in pressure triggers the baroreflex (tachycardia) and the renin system (salt and water retention).

Target: Vascular smooth muscle: potassium channels (minoxidil), soluble guanylyl cyclase (nitroprusside)

## Key points

- Reflex tachycardia and fluid retention are expected: oral direct vasodilators are combined with a beta blocker and a diuretic.
- Hydralazine can cause a lupus-like syndrome.
- Minoxidil causes hypertrichosis and, rarely, pericardial effusion.
- Nitroprusside releases cyanide: the risk grows with prolonged or high-rate infusions and in kidney or liver disease.
- Hydralazine combined with a nitrate is an option in heart failure, and intravenous hydralazine is used in severe hypertension of pregnancy.

## Drugs in this class

- [**Hydralazine** A vasodilator that relaxes the arterioles. It is used intravenously in severe hypertension of pregnancy and, with a… Arteriolar smooth muscle (molecular target not fully established)](https://durgical.com/en/pharmacology/hydralazine/)
- [**Sodium nitroprusside** An intravenous nitric oxide donor for hypertensive emergencies. It works within minutes but releases cyanide, so it… Soluble guanylyl cyclase (activated by the nitric oxide it releases)](https://durgical.com/en/pharmacology/sodium-nitroprusside/)
- [**Minoxidil** A powerful oral vasodilator kept for severe, resistant hypertension. It is given with a beta blocker and a diuretic… ATP-sensitive potassium channel of vascular smooth muscle (SUR2 subunit)](https://durgical.com/en/pharmacology/minoxidil/)

## Others in the class

- Diazoxide
- Dihydralazine

## Test yourself

Think of the answer, then tap the question to see it.

## Why are hydralazine and minoxidil usually given with a beta blocker and a diuretic?

The fall in pressure activates the baroreflex (tachycardia, angina) and the renin-angiotensin-aldosterone system (salt and water retention); the other two drugs block these responses.

## Which toxic product does sodium nitroprusside release?

Cyanide, which the body turns into thiocyanate; both can accumulate with long infusions or organ failure.

## Which direct vasodilator causes excess body hair, and how does it act?

Minoxidil: its sulfate metabolite opens ATP-sensitive potassium channels in arteriolar smooth muscle.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - HYDRALAZINE HYDROCHLORIDE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=336b11da-e592-4386-921f-3c136d754f2a) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - SODIUM NITROPRUSSIDE injection, solution, concentrate](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=6fbfb86c-da8d-4741-b1e0-7000e2bbad5f) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - MINOXIDIL tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0354a7f5-5917-44dc-9963-411104008cb5) (DailyMed (U.S. National Library of Medicine), 2026); Farmacología básica y clínica (Katzung), cap. 11 (McGraw Hill, 2025).

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Representative model, for educational purposes only.

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