Aldose reductase inhibitor: A class of drugs used to prevent eye and nerve damage in diabetes. Aldose reductase is an enzyme that is normally present in the eye and in many other parts of the body. It helps change glucose into a sugar alcohol called sorbitol. Too much sorbitol trapped in eye and nerve cells can damage these cells, leading to retinopathy (retinal disease) and neuropathy (nerve disease). Drugs that prevent or slow the action of aldose reductase may represent a means to prevent or delay these complications of diabetes.