What is Mannitol?

Mannitol is the generic name for a drug used as an osmotic diuretic and a mild renal vasodilator. It is typically administered intravenously, but can also be taken orally, depending on the purpose. Intravenously, it is used to treat excessive intracranial pressure, oliguria, and to expand openings in the blood-brain barrier. Orally, this substance is used a sweetening agent in confections for people with diabetes and, in higher concentrations, as a laxative for children. When it is called a “baby laxative,” this is usually a slang term referencing its use in cutting methamphetamines, heroine, or other drugs used recreationally.

Chemically, mannitol is composed of carbon, hydrogen, and multiple hydroxyl groups (C₆H₈(OH)₆) to make a sugar alcohol. It is prepared in a solution with water at concentrations or 5, 10, 15, 20, and 25%. This substance is not permeable, meaning it will not pass through a cell membrane the way many other drugs will. Because it is prone to making a solution acidic, sodium bicarbonate may be added to adjust the pH.

As an osmotic diuretic, mannitol raises the osmolarity in the nephron of the kidney so that more water is taken out of the body to be excreted. Nephrons are small units in the kidney that absorb nutrients, dump waste, and regulate the balance of water to soluble substances in the body to create urine. Osmolarity measures the amount of solute, such as sodium, to solvent, water. If the body has too little water, the nephron will not allow much water to pass out of the body, but if the body has too much water, it will pass water and sodium out to be excreted with other wastes as urine. Likewise, if there is a high concentration of solute in the nephron, the body will try to achieve a balance by letting water enter through osmosis.

Mannitol passes into the nephron and raises the osmolarity so more water leaves the body. This is how the drug is used in its capacity to decrease intracranial pressure (ICP), or the pressure between the cranium, the tissue of the brain, and the cerebrospinal fluid. Through drawing water out of the body, this drug lowers the volume of extracellular fluid, relieving pressure. This treatment plan, however, has been subject to doubt as to its true effectiveness. The drug’s diuretic property also helps open the blood-brain barrier by shrinking the cells that compose the barrier, so there are larger openings for blood to pass through.

It is important for people who use this substance to be aware of its effects on the body’s sodium and water, so that severe dehydration and hypovolemia may be avoided and preexisting conditions such as hyponatremia aren’t aggravated. The patient should have a heart healthy enough to sustain temporary decrease in extracellular fluid and should not receive blood within a short time period of taking the drug unless absolutely necessary. If it is necessary, sodium chloride should be added to the solution to prevent pseudoagglutination.