Pharmacology · Direct vasodilators
Sodium nitroprusside !
An intravenous nitric oxide donor for hypertensive emergencies. It works within minutes but releases cyanide, so it needs close monitoring.
Updated 2026-11-01
Also called: Nitroprusside
Mechanism of action
Nitroprusside is an iron complex carrying cyanide groups and a nitroso group. In red cells it reacts with haemoglobin and releases nitric oxide, which activates soluble guanylyl cyclase in vascular smooth muscle; cGMP rises and both arteries and veins relax, lowering afterload and preload. The cyanide released is converted to thiocyanate by the mitochondrial enzyme rhodanese.
- Nitroprusside reacts with haemoglobin and releases nitric oxide
- Nitric oxide activates guanylyl cyclase; cGMP rises
- Arteries and veins relax: afterload and preload fall
- Released cyanide is converted to thiocyanate
Pharmacokinetics
- Distribution
- Distributes in the extracellular space
- Metabolism
- Reaction with haemoglobin releases nitric oxide and cyanide; rhodanese turns cyanide into thiocyanate using thiosulfate
- Elimination
- Thiocyanate is cleared by the kidneys, with a half-life of about 3 days that lengthens in kidney failure (label)
- Onset and duration
- Within a minute or two; the effect fades almost as fast when the infusion stops (label)
Indications
- Hypertensive crisis, for immediate reduction of blood pressure Arterial hypertension →
- Controlled hypotension to reduce bleeding during surgery
- Acute heart failure Heart failure →
Adverse effects
Common
- Excessive hypotension
- Nausea, sweating and restlessness from a too-rapid fall in pressure
- Headache and palpitations
- Bradycardia or tachycardia
Contraindications
- Compensatory hypertension (aortic coarctation, arteriovenous shunting)
- Controlled hypotension in surgery with inadequate cerebral circulation or in moribund patients
- Congenital (Leber) optic atrophy or tobacco amblyopia
- Acute heart failure with reduced peripheral resistance, such as high-output failure in sepsis
Key interactions
- Other antihypertensives
- Additive fall in blood pressure
- Ganglionic blockers and inhaled anaesthetics
- Stronger hypotensive effect
What is monitored
- Continuous blood pressure (invasive is usual)
- Acid-base status, lactate and venous oxygen for cyanide toxicity
- Thiocyanate in long infusions or kidney failure
- Methaemoglobin if oxygen delivery seems impaired
Administration and safety
- Not for direct injection: always diluted in a dextrose solution and given by infusion pump.
- Protect the bag from light with an opaque cover; the drip chamber and tubing do not need covering.
- The diluted solution has a faint brownish tint; discard it if it turns blue, green or bright red.
- No other drugs are added to or run in the same solution.
- Only where continuous blood pressure monitoring and trained staff are available.
Nursing care
- Before: an arterial line and an infusion pump are ready; baseline acid-base status and kidney function.
- During: blood pressure continuously; a fall that is too fast can cause cerebral, cardiac or renal ischaemia.
- Watch for signs of cyanide toxicity: rising lactate, metabolic acidosis, confusion, air hunger.
- Watch for thiocyanate toxicity (tinnitus, miosis, hyperreflexia) in long infusions or kidney failure.
- A longer-acting oral antihypertensive is started so the infusion can be stopped as soon as possible.
Pregnancy and breastfeeding
Nitroprusside crosses the placenta, and fetal cyanide levels follow maternal levels in animal studies; it is used in pregnancy only when the benefit clearly outweighs the risk.
Test yourself
Think of the answer, then tap the question to see it.
How does sodium nitroprusside relax vessels?
It releases nitric oxide, which activates soluble guanylyl cyclase; the rise in cGMP relaxes arterial and venous smooth muscle.
Which toxicity limits long or high-rate nitroprusside infusions, and how does it show?
Cyanide toxicity: metabolic (lactic) acidosis, bright red venous blood (cells cannot extract oxygen), confusion and air hunger.
Why is the infusion bag covered?
Nitroprusside is degraded by light, so the bag is protected with an opaque cover.
Which metabolite accumulates in kidney failure during nitroprusside use?
Thiocyanate, which is cleared by the kidneys and causes tinnitus, miosis and hyperreflexia when it accumulates.
Related drugs
Related conditions
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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 - SODIUM NITROPRUSSIDE injection, solution, concentrate (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025); Farmacología básica y clínica (Katzung), cap. 11 (McGraw Hill, 2025).
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Representative model, for educational purposes only.