The primary active constituent in Natra Flex is Natra Health's unique BC421; a proprietary, balanced blend of Boswellia, Capsaicin & MSM. Boswellia serrata is a deciduous tree native to India. Its gum resin has been sued topically for centuries. Capsaicin is a component of Cayenne pepper whilst MSM is a course of biological sulphur. Independent field tests showed that 75% of subjects found BC421 to be highly effective.