What Are Osmotic Diuretics?

Osmotic diuretics are a class of medications that cause patients to have increased urine production. These drugs work by shifting the balance of water between different parts of the body. The different osmotic diuretics include mannitol, isosorbide, urea, and glycerin. In general these medications have a less potent effect compared to some of the other types of diuretics. Although these medications are typically well tolerated, patients with certain underlying diseases should not take them.

The mechanism of action of osmotic diuretics is to alter how the kidney filters the blood and produces urine. In general, osmotic agents exert their effects because their presence increases the amount of substances dissolved in the blood. With high concentrations of osmotic agents in the body, water will flow from inside the cells and into the bloodstream in an effort to normalize the relative concentration of substances in the intracellular and extracellular spaces in the body. This results in more fluid going to the kidney to be filtered, causing increased urine production. Additionally, the presence of osmotic agents in the fluid being filtered by the kidney can result in less water being reabsorbed into the bloodstream, resulting in increased urination.

There are four different diuretics that are typically used. One of the most important is a chemical species called mannitol, which is a small molecule that is not digested and does not participate in any chemical reactions within the body. The other two commonly used agents include glycerin and isosorbide. Urea, another osmotic diuretic, is not used in the U.S. but can be prescribed in other countries around the world.

Overall, the osmotic diuretics are considered to be less potent medicines compared to some of the other classes of diuretics. They cause patients to urinate less as compared to some of the other diuretics. Therefore, while they are sometimes used solely to increase urine output, they are also used in a few other ways. For example, mannitol can be given to patients who have an increased intracranial pressure in an effort to decrease this pressure by causing some of the fluid in the brain to flow into the bloodstream.

Common side effects of diuretics can include headache, vomiting, and nausea. Prolonged use of the medication could decrease the concentration of sodium in the blood, leading to a condition called hyponatremia. When mild, this condition causes non-specific symptoms; severe hyponatremia can cause seizures, however.

Not all patients should be given osmotic diuretics. Giving osmotic diuretics increases the volume of blood circulating around the body. In patients who already have too much fluid in their bodies, for example in patients with end-stage kidney disease or congestive heart failure, the medications should not be given because they could worsen the ramifications of these underlying disease processes. Patients who do not produce urine should also not be given the medications.