Per Package Insert (25% vial)-
"Crystals may form in Mannitol Injection, especially if the solution is exposed to low temperatures. If crystallization occurs, warm the vial in water at 80°C and periodically shake vigorously to dissolve the crystals."
While the above recommends water for rewarming, other resources such as ASHP Injectable Drug Information book suggests, "The use of a water bath is not recommended."
Rewarming comes with its own risks to STRONGLY consider. One such risk is pressure build up in the vial and explosion. This is an unfortunate circumstance we've experienced (without harm, thank heavens!) after heating with a medication hot plate, a process previously done a plethora of times without negative consequence.
How are your sites heating mannitol vials safely?