Glycerin (glycerol) is 3-carbon alcohol naturally occurring in the human body. It is the structural backbone triacylglycerol molecules, and can also be converted to a glycolytic substrate for subsequent metabolism. Glycerin is a colorless, odorless, viscous, sweet-tasting liquid. The FDA classifies glycerol as "generally recognized as safe". Glycerin is used in the pharmaceutical industry as a sweetener in syrups, lozenges, and as an excipient in eyewash solutions. As an individual prescription product, glycerin has uses as a hyperosmotic, osmotic diuretic, and ophthalmic agent. It may be used as an eye drop in the treatment of glaucoma to reduce intraocular pressure, as a solution or suppository for short-term treatment of constipation, to evacuate the bowel prior to a colonoscopy, and in some ocular surgeries. It may be given intravenously to reduce pressure inside the brain and used externally on the skin as a moisturizer. Glycerin has many other uses in the agricultural, food and pharmaceutical industry.