Mannitol !

A sugar alcohol given by IV infusion that draws water out of the brain and eye. Used for raised intracranial and intraocular pressure.

Updated 2026-11-01

Class: Osmotic diuretics
Target: No protein target: osmotic effect in plasma and tubular fluid
ATC code: B05BC01
WHO Model List of Essential Medicines

Mechanism of action

Mannitol stays outside cells and in the blood it raises osmolality, pulling water out of brain tissue and the eye into the plasma. The glomerulus filters it freely and the tubule barely reabsorbs it, so it holds water in the lumen and causes an osmotic diuresis with loss of water and some sodium.

Mannitol raises plasma osmolality
Water moves from brain and eye into the blood
Filtered mannitol holds water in the tubule
Osmotic diuresis; intracranial and eye pressure fall

Pharmacokinetics

Absorption: Given IV only for these uses (it is not absorbed well by mouth)
Distribution: Stays mainly in the extracellular fluid
Metabolism: Only a small amount is metabolised
Elimination: Filtered by the glomeruli and excreted unchanged in the urine
Half-life: About half an hour to 2.5 hours with normal kidneys; around 36 hours in kidney failure

Indications

Raised intracranial pressure and cerebral oedema Traumatic brain injury →
Raised intraocular pressure, for example before eye surgery or in acute angle-closure glaucoma Acute angle-closure glaucoma →

Adverse effects

Common

Fluid and electrolyte imbalance: high or low sodium and potassium
Headache, nausea, dizziness
Phlebitis at the infusion site
Thirst and dehydration

Contraindications

Anuria
Severe hypovolaemia
Active intracranial bleeding, except during craniotomy
Hypersensitivity to mannitol

Key interactions

Nephrotoxic drugs or other diuretics: Higher risk of kidney failure
Neurotoxic drugs: More CNS toxicity
Lithium: Risk of lithium toxicity; levels need close monitoring
Drugs sensitive to potassium changes (e.g. digoxin): Arrhythmias if potassium shifts

What is monitored

Serum osmolality and sodium
Potassium and kidney function
Urine output
Heart and lung status (signs of fluid overload)
Neurological status and intracranial pressure

Administration and safety

IV infusion only, preferably through a central line: the hypertonic solution irritates peripheral veins and extravasation can cause compartment syndrome.
Use an administration set with an in-line filter, because crystals can form.
Inspect before use: crystals appear in the cold; follow the product's warming instructions and give only a clear solution.
Do not give through the same set as blood products, and do not transfer to PVC bags (a precipitate can form).
Before starting, check kidney, heart and lung status and correct fluid and electrolyte problems.

Nursing care

Measure urine output closely (often with a urinary catheter); report low urine output.
Check serum osmolality, sodium and potassium as ordered during therapy.
Watch for fluid overload: breathlessness, crackles, rising pressure.
Monitor neurological status and watch for rebound of intracranial pressure hours later.
Check the infusion site often for redness, swelling or leakage.

Pregnancy and breastfeeding

Used only when the benefit clearly outweighs the risk; data in pregnancy are limited.

Test yourself

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

How does mannitol cause diuresis?

It is filtered by the glomerulus and not reabsorbed, so it holds water in the tubule lumen by osmosis.

Why is mannitol given with a filter?

Mannitol solutions can form crystals, especially in the cold, and the filter keeps crystals out of the vein.

What can happen if mannitol leaks out of the vein?

Local irritation and swelling and, in severe cases, compartment syndrome.

Can you name two contraindications of mannitol?

Anuria and severe hypovolaemia; also active intracranial bleeding outside a craniotomy.

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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 - MANNITOL injection, solution (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).

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