L-CYSTEINE is a white crystalline powder or crystals that is freely soluble in water and practically insoluble in ethanol. L-CYSTEINE contains not less than 98.0 percent and not more than 101.0 percent of L-CYSTEINE (C6H10N2O2S) on a dried basis. L-CYSTEINE is commonly used in the pharmaceutical industry as an excipient due to its antioxidant properties. It helps protect active pharmaceutical ingredients and finished drug products from degradation. L-CYSTEINE also helps promote the bioavailability of certain drugs by facilitating their absorption.