Malonic acid (propanedioic acid) is the archetypal example of a competitive inhibitor: it acts against succinate dehydrogenase (complex II) in the respiratory electron transport chain. Malonic acid is found to be associated with malonyl-CoA decarboxylase deficiency which is an inborn error of metabolism.

Monoethyl malonic acid belongs to the class of organic compounds known as dicarboxylic acids and derivatives. These are organic compounds containing exactly two carboxylic acid groups. Monoethyl malonic acid exists as a solid soluble (in water) and a weakly acidic compound (based on its pKa).

Harmane (harman) is a bio-active β-Carboline and monoamine oxidase inhibitor found in coffee and tobacco smoke.