Combined Effect of Drug | Synergistic Effect | Additive Effect | Superaadditive Effect of Drugs

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Synergism comes from the Greek word "synergies" meaning working together. It refers to the interaction between two or more "things" when the combined effect is greater than if you added the "things" on their own

In toxicology, synergism refers to the effect caused when exposure to two or more chemicals at as time results in health effects that are greater than the sum of the effects of the individual chemicals.

Additive Effect - This action occurs when the combined effect of two or more chemicals is equal to the sum of the effect of each agent given alone, they do not interact in a direct way for example:

2 + 2 = 4

This effect is the most common when two chemicals are given together.

Potentiation - This effect results when one substance that does not normally have a toxic effect is added to another chemical, it makes the second chemical much more toxic; for example:

Antagonism - Antagonism is the opposite of synergism. It is the situation where the combined effect of two or more compounds is less toxic than the individual effects; for example:

Antagonistic effects are the basis of many antidotes for poisonings or for medical treatments. For example, ethyl alcohol (ethanol) can antagonize the toxic effects of methyl alcohol (methanol) by displacing it from the enzyme that oxidizes the methanol

In comparison, a synergistic effect is a situation where the combined effect of two chemicals is much greater than the sum of the effects of each agent given alone, for example:

There are various examples including:

(a) Carbon tetrachloride and ethanol (ethyl alcohol) are individually toxic to the liver, but together they produce much liver injury than the sum of their individual effects on the liver.

(b) The much higher incidence of lung cancer resulting from occupational exposure to asbestos in smokers (compared to exposed non-smokers).

(c) The toxicity of some insecticides notably pyrethrin (from chrysanthemums) and synthetic pyrethrins (pyrethroids) can be increased many times by the addition of compounds which themselves are not insecticides.

(d) Barbiturate drugs have a greater effect on the central nervous system (CNS) by causing CNS depression when taken with general anaesthetics, alcohol (acute consumption) narcotic analgesic (pain reliever) and other sedative-hypnotic drugs.

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