Preventing or treating excess body water in certain kidney conditions, reducing swelling of the brain, or reducing pressure in the eye. It may also be used for other conditions as determined by your doctor.
Mannitol is an osmotic diuretic. It works by increasing the amount of fluid excreted by the kidneys and helps the body to decrease pressure in the brain and eyes.

Do not use mannitol if:
you are allergic to any ingredient in mannitol
you have a history of heart failure
you have decreased or absent production of urine due to severe kidney disease, certain severe lung problems (eg, pulmonary congestion or pulmonary edema), bleeding in the brain, or severe dehydration
Contact your doctor or health care provider right away if any of these apply to you.

Before using mannitol:
Some medical conditions may interact with mannitol. Tell your doctor or pharmacist if you have any medical conditions, especially if any of the following apply to you:
if you are pregnant, planning
to become pregnant, or are breast-feeding
if you are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
if you have allergies to medicines, foods, or other substances
if you have swelling, kidney problems, or heart problems (eg, congestive heart failure)
Some medicines may interact with mannitol. However, no specific interactions with mannitol are known at this time.
Ask your health care provider if mannitol may interact with other medicines that you take. Check with your health care provider before you start, stop, or change the dose of any medicine.

How to use mannitol:
Use mannitol as directed by your doctor. Check the label on the medicine for exact dosing instructions.
Mannitol is usually administered as an injection at your doctor's office, hospital, or clinic. If mannitol contains particles or is discolored, or if the vial is cracked or damaged in any way, do not use it.
Keep this product, as well as syringes and needles, out of the reach of children and pets. Do not reuse needles, syringes, or other materials. Ask your health care provider how to dispose of these materials after use. Follow all local rules for disposal.

If you miss a dose of mannitol, contact your doctor right away.
Ask your health care provider any questions you may have about how to use mannitol.
Important safety information:
Mannitol may cause dizziness. These effects may be worse if you take it with alcohol or certain medicines. Use mannitol with caution. Do not drive or perform other possibly unsafe tasks until you know how you react to it.

Tell your doctor immediately if you have difficulty urinating or experience extreme dizziness.
Lab tests, including blood electrolytes, kidney function, lung function, heart function, and blood counts, may be performed to monitor your progress or to check for side effects. Be sure to keep all doctor and lab appointments.
Use mannitol with caution in the elderly; they may be more sensitive to its effects.
Mannitol should be used with extreme caution in children younger than 12 years old; safety and effectiveness in these children have not been confirmed.

pregnancy and breast-feeding: It is not known if mannitol can cause harm to the fetus. If you become pregnant, contact your doctor. You will need to discuss the benefits and risks of using mannitol while you are pregnant. It is not known if this medicine is found in breast milk. If you are or will be breast-feeding while you use mannitol, check with your doctor. Discuss any possible risks to your baby.

Possible side effects of mannitol:
All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most common side effects persist or become bothersome:
Increased urination; nausea; runny nose; vomiting.
Seek medical attention right away if any of these severe side effects occur:
Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); blurred vision; chest pain; chills or fever; confusion; decreased alertness; difficulty urinating; extreme dizziness; extreme thirst or dry mouth; fast or irregular heartbeat; headache; muscle cramps; pain, redness, or swelling at the injection site; weakness.

Source: drugs.com