Mannitol

Mannitol (D-mannitol) is a six-carbon sugar alcohol found naturally in seaweed, mushrooms and the manna ash tree. Industrially it is produced by catalytic hydrogenation of fructose or glucose-fructose syrups, yielding a white, odourless crystalline powder with approximately 50 % the sweetness of sucrose.

In pharmaceutical formulation, mannitol's stand-out property is its non-hygroscopic nature — it does not absorb atmospheric moisture even at elevated relative humidity. This makes it the excipient of choice for moisture-sensitive APIs that would degrade on contact with water. Its endothermic heat of dissolution (negative ΔH) produces a pleasant cooling sensation in the mouth, making it the standard sweetener-filler in chewable and orally disintegrating tablets (ODTs).

For lyophilised (freeze-dried) parenterals, mannitol crystallises during the freezing step to form a mechanically rigid cake that supports rapid reconstitution with minimal shrinkage. It is the single most common bulking agent in injectable lyophilisates, used alongside amorphous cryoprotectants such as sucrose or trehalose.

Beyond excipient use, mannitol itself is a pharmacologically active substance: intravenous mannitol 20 % is a first-line osmotic diuretic for managing raised intracranial pressure, cerebral oedema and acute oliguric renal failure. Inhaled dry-powder mannitol is also used as a diagnostic agent in the bronchial provocation (mannitol challenge) test for airway hyper-responsiveness in asthma.