In pharmacology and biochemistry, a small molecule is an organic compound that is not a polymer. Biopolymers such as nucleic acids, proteins, and polysaccharides (such as starch or cellulose) are not small molecules, although their constituent monomers—ribo- or deoxyribonucleotides, amino acids, and monosaccharides, respectively—are often considered to be. Very small oligomers are also usually considered small molecules, such as dinucleotides, peptides such as the antioxidant glutathione, and disaccharides such as sucrose.
While small molecules almost always have a lower molecular weight than biopolymers, a very small protein with a defined fold, such as the artificial ten-amino-acid protein chignolin, can indeed be smaller than some exceptionally large small molecules such as triglycerides.
Small molecules can have a variety of biological functions, serving as cell signalling molecules, as tools in molecular biology, as drugs in medicine, and in countless other roles. These compounds can be natural (such as secondary metabolites) or artificial (such as antiviral drugs); they may have a beneficial effect against a disease (such as FDA approved drugs) or may be detrimental (such as teratogens and carcinogens).