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Interactions
Humans are normally exposed to several chemicals at one time rather than to an individual chemical. Medical treatment and environment exposure generally consists of multiple exposures. Examples are:
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|  | hospital patients on the average receive 6 drugs daily
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| | home influenza treatment consists of aspirin, antihistamines, and cough syrup taken simultaneously
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| | drinking water may contain small amounts of pesticides, heavy metals, solvents, and other organic chemicals
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| | air often contains mixtures of hundreds of chemicals such as automobile exhaust and cigarette smoke
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| | gasoline vapor at service stations is a mixture of 40-50 chemicals
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Normally, the toxicity of a specific chemical is determined by the study of animals exposed to only one chemical. Toxicity testing of mixtures is rarely conducted since it is usually impossible to predict the possible combinations of chemicals that will be present in multiple-chemical exposures.
Xenobiotics administered or received simultaneously may act independently of each other. However, in many cases, the presence of one chemical may drastically affect the response to another chemical. The toxicity of a combination of chemicals may be less or it may be more than would be predicted from the known effects of each individual chemical. The effect that one chemical has on the toxic effect of another chemical is known as an interaction.
 
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