Glycosides are present in many drugs and spices. They are also the chief constituents of animal tissues. Aglycones are methanol, glycerol, sterol, or phenol. Glycosides play a very important role in medicine because of their action in heart (cardiac glycosides). Examples are the derivatives of digitals and ouabain. Ouabain is a very good inhibitor of Na⁺—K⁺—ATPase of plasma membranes. Examples of other glycosides are antibiotics such as streptomycin. Phlorizin is a glycoside that inhibits the transport of sugars across the mucosal cells of small intestine.