An osmotic diuretic given by drip
Mannitol
A specialist medicine given by drip to pull excess fluid out of tissues, used to reduce raised pressure in the brain or eye.
What is Mannitol?
Mannitol is a specialist medicine, a sugar-alcohol given by drip into a vein, that draws excess fluid out of the body's tissues. It is used mainly to reduce raised pressure inside the brain, for example after a serious head injury or during brain surgery, and to lower raised pressure in the eye. It is also used in some kidney situations. It is given in hospital by a specialist team and needs careful monitoring of fluids, blood salts and the kidneys. It is given as an infusion.
Education and reference only. This is a plain-language guide to Mannitol — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Mannitol is a prescription-only medicine known as an osmotic diuretic. It is a type of sugar-alcohol that, when given into a vein, draws water out of swollen tissues and into the bloodstream, where it is then passed out as urine. This makes it useful for reducing dangerously raised pressure inside the skull, for example after a head injury or during neurosurgery, and for lowering raised pressure within the eye. It is sometimes used in certain kidney situations too. It is given by infusion in hospital under close specialist supervision.
How it works
Mannitol works by raising the concentration of the blood so that water is pulled from areas of swelling, such as the brain or the eye, back into the bloodstream. This reduces the swelling and lowers the raised pressure. The extra fluid drawn into the blood is then removed by the kidneys as urine, which is why it also increases how much urine is made. Because it shifts fluid and salts around the body quickly, it has to be given carefully, with close attention to the person's fluid balance, blood salts and kidney function.
Company & origin
Originated / developed by: Generic (long-established).
A long-established specialist medicine used in the UK in hospital to reduce raised pressure in the brain or eye and in some kidney situations.
Practical use
How to take Mannitol
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- It is given by infusion into a vein in hospital by a specialist team; you do not take it yourself.
- Your fluid balance, blood salts and kidney function are checked closely before, during and after treatment.
- The infusion is often filtered and warmed if needed, because the medicine can form crystals if it gets cold.
- Tell the team if you feel unwell, very thirsty, breathless or notice swelling during treatment.
- The amount and rate are carefully adjusted to your response and your kidney and heart function.
Weighing it up
Advantages & disadvantages of Mannitol
Advantages
- Quickly reduces dangerously raised pressure in the brain or the eye.
- Well established, with long experience of its use in emergency and surgical settings.
- Acts on swelling by drawing fluid back into the bloodstream, where it is removed as urine.
Disadvantages
- Has to be given by infusion in hospital under close supervision.
- Can cause dehydration or, conversely, fluid overload, and can upset the blood salts.
- Needs careful monitoring and is used with caution in people with heart or kidney problems.
Practical use
Good to know
Mannitol is a hospital medicine given by drip, often in emergency or operating-theatre settings, so it is always closely supervised. Because it moves fluid and salts around the body quickly, careful monitoring of fluid balance, blood salts and the kidneys is essential, and it is used with particular caution in people with heart or kidney problems, who could become overloaded with fluid or develop salt imbalances. It is filtered before use because it can form crystals if it gets cold. People are watched closely for signs of dehydration or fluid overload during and after treatment, and the infusion is adjusted to the person's response.
Who should not take it / use with caution
- It is generally avoided in people whose kidneys have stopped making urine despite a trial of it.
- It is used with great caution, or avoided, in people with severe heart failure or fluid overload.
- It is not suitable in certain bleeding within the skull or severe dehydration, as judged by the specialist team.
Monitoring
- Close monitoring of fluid balance, including how much urine is made.
- Regular blood tests for blood salts and kidney function during treatment.
- Watching for signs of dehydration or fluid overload and adjusting the infusion accordingly.
Side effects
- Changes in fluid balance, leading to dehydration or fluid overload if not carefully managed.
- Disturbances in blood salts, headache, nausea or thirst.
- Rarely, kidney problems, especially with high amounts, or reactions at the infusion site.
Key interactions
- Used carefully with other diuretics and medicines that affect blood salts or fluid balance.
- It can affect the levels of some medicines, including certain heart and kidney medicines, so these are reviewed.
- Tell the team about all medicines, as the combination is checked alongside fluid and salt monitoring.
Available as: A solution given by infusion into a vein.
Answers
Mannitol: frequently asked questions
What is mannitol used for?
It is used mainly to reduce raised pressure inside the brain, for example after a head injury or during brain surgery, and to lower raised pressure in the eye, as well as in some kidney situations.
How does it work?
It draws excess water out of swollen tissues, such as the brain or eye, back into the bloodstream, reducing the pressure; the extra fluid is then passed out as urine.
How is it given?
It is given by infusion into a vein in hospital by a specialist team, often in emergency or operating-theatre settings.
Why is so much monitoring needed?
It moves fluid and salts around the body quickly, so fluid balance, blood salts and the kidneys are checked closely to avoid dehydration, fluid overload or salt imbalance.
Is it safe if I have heart or kidney problems?
It is used with particular caution in people with heart or kidney problems, who could become overloaded with fluid, so the team monitors closely and adjusts treatment.
Authoritative sources
- BNF
- NICE CKS
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