Reprinted with permission from the Federal Judicial Center's 2000 Reference Manual on Scientific Evidence.

Toxicology classically is known as the science of poisons. A modern definition is the study of the adverse effects of chemicals on living organisms. Although it is an age-old science, toxicology has only recently become a discipline distinct from pharmacology, biochemistry, cell biology, and related fields.

There are three central tenets of toxicology. First, the dose makes the poison this implies that all chemical agents are intrinsically hazardous whether they cause harm is only a question of dose. Even water, if consumed in large quantities, can be toxic. Second, each chemical agent tends to produce a specific pattern of biological effects that can be used to establish disease causation. Third, the toxic responses in laboratory animals are useful predictors of toxic responses in humans. Each of these tenets, and their exceptions, are discussed in greater detail in this reference guide.

The science of toxicology attempts to determine at what doses foreign agents produce their effects. The foreign agents of interest to toxicologists are all chemicals (including foods) and physical agents in the form of radiation, but not living organisms that cause infectious diseases.

The discipline of toxicology provides scientific information relevant to the following questions:
1. What hazards does a chemical or physical agent present to human populations
or the environment?
2. What degree of risk is associated with chemical exposure at any given
dose?

Toxicological studies, by themselves, rarely offer direct evidence that a disease in any one individual was caused by a chemical exposure. However, toxicology can provide scientific information regarding the increased risk of contracting a disease at any given dose and help rule out other risk factors for the disease. Toxicological evidence also explains how a chemical causes a disease by describing metabolic, cellular, and other physiological effects of exposure.