The fruit of most species of Capsicum contains capsaicin . Capsaicin is a chemical that can produce a strong burning sensation in the mouth ( and , if not properly digested , anus ) of the unaccustomed eater . The fruit of most species of Capsicum contains capsaicin ( methyl vanillyl nonenamide ) , a lipophilic chemical that can produce a strong burning sensation in the mouth of the unaccustomed eater .