In chemistry, antagonism is a phenomenon wherein two or more agents in combination have an overall effect that is less than the sum of their individual effects.
The word is most commonly used in this context in biochemistry and toxicology: interference in the physiological action of a chemical substance by another having a similar structure. For instance, a receptor antagonist is an agent that reduces the response that a ligand produces when the receptor antagonish binds to a receptor on a cell. An example of this is the interleukin-1 receptor antagonist. The opposite of antagonism is synergy. It is a negative type of synergism.