Is 50 ml the maximum safest dose of mannitol? What happens if there's an infusion of more than that?

It is best to discuss this with patient's doctor. This is the dosing schedule ... depending on the weight of the patient I think he may get away with any side effects if the high dose was only done once.

Mannitol Dosing Information

Usual Adult Dose for Oliguria:

Renal function test dose prior to initiation of treatment: 0.2 g/kg IV over 3 to 5 min resulting in a urine flow of at least 30 to 50 mL/hr. A second test dose may be administered if the urine flow does not increase. If no response is seen following the second test dose, the patient should be reevaluated.

Treatment: 300 to 400 mg/kg (21 to 28 g for a 70 kg patient) or up to 100 g of 15% to 20% solution IV once. Treatment should not be repeated in patients with persistent oliguria.

Prevention (for use during cardiovascular and other types of surgery): 50 to 100 g IV. usually a 5 , 10, or 20% solution is used depending on the fluid requirements of the patient.

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Usual Adult Dose for Cerebral Edema:

0.25 to 2 g/kg as a 15 to 20% solution IV over at least 30 min administered not more frequently than every 6 to 8 hrs.

To yield a satisfactory reduction in intracranial pressure, the osmotic gradient between the blood and cerebrospinal fluid should remain approximately 20 mOsmol.

In small &/or debilitated patients 500 mg/kg may be sufficient.

http://www.drugs.com/mtm/mannitol.html