Drug Interactions - Synergy and Antagonism

Synergy and Antagonism

When the interaction causes an increase in the effects of one or both of the drugs the interaction is called a synergistic effect. An “additive synergy” occurs when the final effect is equal to the sum of the effects of the two drugs (Although some authors argue that this is not true synergy). When the final effect is much greater than the sum of the two effects this is called enhanced synergy. This concept is recognized by the majority of authors, although other authors only refer to synergy when there is an enhanced effect. These authors use the term “additive effect” for additive synergy and they reserve use of the term “synergistic effect” for enhanced synergy. The opposite effect to synergy is termed antagonism. Two drugs are antagonistic when their interaction causes a decrease in the effects of one or both of the drugs.

Both synergy and antagonism can both occur during different phases of the interaction of a drug with an organism, with each effect having a different name. For example, when the synergy occurs at a cellular receptor level this is termed agonism, and the substances involved are termed agonists. On the other hand, in the case of antagonism the substances involved are known as inverse agonists. The different responses of a receptor to the action of a drug has resulted in a number of classifications, which use terms such as “partial agonist”, “competitive agonist” etc. Concepts which have a fundamental application in the pharmacodynamics of these interactions. The proliferation of existing classifications at this level, along with the fact that the exact reaction mechanisms for many drugs are not well understood means that it is almost impossible to offer a clear classification for these concepts. It is even likely that many authors would misapply any given classification.