A biologically active substance is a compound that engages in and influences life phenomena in a small amount. Biologically active substances include vitamins and coenzymes, hormones, antibiotics, neurotransmitters, and cytokines, for example. They mainly act as regulators of biological functions and are also being studied as candidate compounds for pharmaceuticals. Many of the bioactive compounds have actions in the body that are essential to health. They are being studied in the prevention of cancer, heart disease, and other diseases. Examples include lycopene, resveratrol, lignan, tannins, and indoles.