Barley contains approximately 5-8% gluten on average and is one of three grains that are gluten-based. Gluten is found in the plant's grains, or seeds, which are used in breads, beer and malted products. It is not clear whether or not barley grass, often used in supplements, contains gluten.
Barley contains the gluten protein hordein, which causes various issues for those with celiac disease or gluten sensitivity. Barley malt is used in products, such as candy bars, as a sweetener, but it is not always labeled by food manufacturers as the FDA does not require this disclosure. Therefore, these products are not gluten-free, and those with gluten issues should carefully read labels.