One of the most fundamental tenets of toxicology is that "the dose determines the poison." This simple phrase provides the basis for the belief that all agents—chemicals and physical phenomena that are capable of producing some effect—have the potential to cause toxicity. Whether toxicity actually occurs is principally a matter of dose: the greater the exposure to a given agent, the more pronounced or severe the response of a cell or organism. While this is obvious for well-known poisons such as cyanide, arsenic, lead, and pesticides, it is also true that essential substances such as vitamins, minerals, and even oxygen are toxic at excessive doses.

The tenet that the dose determines the poison provides the basic framework for how toxicologists assess the "hazard potential" of chemical products and materials. The goal of such assessments is to determine the levels of exposure that cause harmful effects, the nature of those effects, and the so-called safe level of exposure. Toxicological testing is designed, therefore, to determine what toxicologists call the dose-response relationship. Investigators attempt to describe how a given chemical affects the body at varying doses ranging from very high to very low. The type of information these studies yield will hopefully lead to a determination of the threshold that separates a safe exposure from the early stages of toxicity. While establishing evidence of a true toxicity threshold is often complicated, the belilef is that such thresholds exist for each harmful effect and that they can be determined from toxicology studies.