Cardiac glycosides are organic compounds containing a glycoside (sugar) that act on the contractile force of the cardiac muscle. Because of their potency in disrupting the function of the heart, most are extremely toxic. These glycosides are found as secondary metabolites in several plants, and in some insects, such as the milkweed butterflies, that consume the plants. Chemically, the aglycone [non-sugar] part of cardiac glycoside is a steroidal moiety.