Toxicology, derived from the Greek word, toxicon, is the scientific study of poisons or toxins. The National Library of Medicine describes toxicology as “the study of the adverse effects of chemicals or physical agents on living organisms.” Understanding how these toxins affect humans and other animals requires studying these basic relationships. It involves observing and reporting symptoms, mechanisms, detection, and treatments of toxic substances, in particular relation to poisoning.


The study and classification of toxic substances goes back two thousand years to Dioscorides (CE 40–90), a Greek physician and pharmacologist who compiled the five-volume encyclopedia on medicinal plants and drugs, De Materia Medica. The Swiss physician and alchemist Theophrastus Philippus Aureolus Bombastus von Hohenheim, better known as Paracelsus (1493–1541), however, is said to be the father of the modern science of toxicology. Paracelsus wrote, “All things are poison, and nothing is without poison, the dose alone makes a thing not a poison.” In other words, if poisoning is to be caused, an exposure to a potentially toxic chemical must result in a dose that exceeds a physiologically determined threshold of tolerance. Smaller exposures do not cause poisoning.

The dose size of a toxin is a crucial factor to consider when evaluating the effects of a toxin. Small quantities of a substance such as strychnine taken daily over an extended period of time might have little to no effect, whereas one large dose in one day can be fatal. In addition, some toxins may only affect a particular species of organism, such as pesticides and antibiotics that kill insects and microorganisms with significantly less harmful effects on humans.

Organisms vary greatly in their tolerance of exposure to chemicals. Even within populations of the same species great variations in sensitivity can exist. In rare cases, some individuals may be extremely sensitive to particular chemicals or groups of similar chemicals, a phenomenon known as hypersensitivity. Organisms are often exposed to a wide variety of potentially toxic chemicals through medicine, food, water, and the atmosphere.

The study of the disruption of biochemical pathways by poisons is an important part of toxicology. Poisons affect normal physiology in many ways, but some of the more common mechanisms involve the disabling of enzyme systems, induction of cancers, interference with the regulation of blood chemistry, and disruption of genetic processes.

Toxic agents may be physical (for example, radiation), biological (for example, venom from a poisonous snake bite), or chemical (for example, arsenic) in nature. In addition, biological organisms may cause disease by invading the body and releasing toxins. An example of this process is tetanus, in which the bacterium Clostridium tetani releases a powerful toxin that travels to the nervous system.

Toxic agents may also cause systemic or organspecific reactions in the body. Cyanide affects the entire body by interfering with the body's capacity to use oxygen. Lead has three specific targets: the central nervous system, the kidneys, and the hematopoietic (blood-cell generating) system. The target is affected by the dose and route of the toxin. For example, the initial effects of a chemical may affect the nervous system; repeated exposure might cause chronic damage to the liver.


As of 2018, toxicology was a discipline with a growing number of disciplines.

Environmental and occupational toxicology

The toxicologist employs the tools and methods of science to understand more completely the consequences of exposure to toxic chemicals. Toxicologists typically assess the relationship between toxic chemicals and environmental health by evaluating such factors as the following:

Medical toxicology

Medical toxicology is recognized as a subspecialty by the American Board of Medical Specialties. Medical toxicologists may work in emergency departments, intensive care units, poison control centers, public health organizations, or pharmaceutical and government research facilities. Their work includes:

Forensic toxicology

Forensic toxicology is the subdiscipline that assists with medicolegal investigations of poisoning, drug use, and suspicious deaths. Forensic toxicologists typically work with samples of hair, urine, blood, or other body fluids, various body tissues, and stomach contents. They may also evaluate or analyze pill bottles, other substance containers, syringes, drinking glasses, or similar objects found at the scene of the investigation. In addition, forensic toxicologists may be called on to serve as expert witnesses in courtroom proceedings.

Role in human health

Humans are exposed to complex mixtures of chemicals, many of which are synthetic and have been either deliberately or accidentally released into the environment. In some cases, people actively expose themselves to chemicals that are known to be toxic, such as those found in cigarettes, alcohol, or recreational drugs. Voluntary exposure to chemicals also occurs when people take medicines to deal with illness, use certain types of cosmetics and hair products, or when they choose to work in an occupation that involves routinely dealing with dangerous chemicals. Most exposures to potentially toxic chemicals are inadvertent and involve living in an environment that is contaminated with small concentrations of pollutants, such as those associated with pesticide residues in food, lead from gasoline combustion, or sulfur dioxide and ozone in the urban atmosphere.

A substance that combats the effects of a poison or toxin.
The absorption of toxic chemicals by the tissues of a living organism.
Dealing with matters of interest to a court of law, whether criminal or civil proceedings.
Gastric lavage—
The act of emptying out the stomach via orogastric or nasogastric tube.

Drugs given to improve health can lead to toxicity even when given in appropriate doses. Conditions such as dehydration and other forms of physiological compromise can make the patient more vulnerable to toxicity. Drugs such as acetaminophen, digoxin, lidocaine, and lithium are common examples of drugs with potentially toxic effects. Interactions of substances in the body may also produce toxic effects. For example, if two central nervous system depressants are taken at once, as in the case of combining alcohol and a benzodiazepine tranquilizer, the effects are additive and could lead to extreme depression of the central nervous system functions.

The healthcare system's role related to toxicology includes education and prevention as well as treatment of both acute and chronic effects of toxins. Such agencies as the Food and Drug Administration (FDA), the National Toxicology Program (NTP), the Agency for Toxic Substances and Disease Registry (ATSDR; part of the CDC), and the Occupational Safety and Health Administration (OSHA) work with health care and industry to offer guidelines and restrictions on the manufacture and use of pharmaceuticals, foods, cosmetics, and other substances.

Common diseases and disorders

Toxicologists have ranked the most commonly encountered toxic chemicals in the United States. In descending order of frequency of encounter, they are as follows:


National Toxicology Program

On November 3, 2016, the U.S. Department of Health and Human Services released the fourteenth Report on Carcinogens. The report is a congressionally mandated, science-based, public health document that the National Toxicology Program (NTP) prepares for the secretary of the U.S. Department of Health and Human Services (HHS). It identifies substances that can pose a cancer hazard for people in the United States. The report pinpoints different chemicals; infectious agents, such as viruses; physical agents, such as x rays and ultraviolent (UV) radiation; mixtures; and other substances that could be deemed a human carcinogen.

Newly reviewed substances on the 2016 report that are known to be human carcinogens include the following viruses: Human immunodeficiency virus type 1 (HIV-1); Human T-Cell lymphotropic virus type 1 (HTLV-1); Epstein-Barr virus (EBV); Kaposi sarcoma-associated herpesvirus (KSHV); and Merkel cell polyomavirus (MCV). Another newly reviewed substance, trichloroethylene (TCE), an industrial solvent, is also a known carcinogen; also, cobalt and cobalt compounds that release cobalt ions in vivo is reasonably anticipated to be a human carcinogen. It is a metallic element and its compounds. These seven newly reviewed substances bring the cumulative total to 248 listings. All five viruses are linked to 20 different types of cancers, and recent studies as of 2018 show a causal association between TCE exposure and an increased cancer risk.

Through the NTP, there is a transparent process using a systematic review that guides the carcinogen report. The report is available on the NTP website.

See also Arsenic ; Benzene ; Benzene and benzene derivatives exposure ; Cadmium ; Cadmium poisoning ; Chemical poisoning ; Epidemiology ; Mercury ; Mercury poisoning .



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Klaasen, Curtis D., and John B. Watkins III. Casarett & Doull's Essentials of Toxicology, 3rd ed. New York: McGraw-Hill Education, 2015.

Kolok, Alan. Modern Poisons: A Brief Introduction to Contemporary Toxicology, Washington, DC: Island Press, 2016.

Marcus, Steven M. Medical Toxicology: Antidotes and Anecdotes. New York: Springer, 2017.

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Division of Toxicology and Environmental Medicine, 4770 Buford Hwy. NE, Atlanta, GA, 30341, (800) CDCINFO,, .

American Association of Poison Control Centers, (800) 222-1222,, .

American College of Medical Toxicology, 10645 N. Tatum Blvd., Ste. 200-111, Phoenix, AZ, 85028, (623) 5336340, Fax: (623) 533-6520,, .

American College of Toxicology, 9650 Rockville Pike, No. 3408, Bethesda, MD, 20814, (301) 634-7840, Fax: (301) 634-7852, .

Food and Drug Administration, 10903 New Hampshire Ave., Silver Spring, MD, 20993, (888) 463-6332, .

National Toxicology Program, PO Box 12233, MD K2-05, Research Triangle Park, NC, 27709, (919) 541-3419, .

Society of Forensic Toxicologists, One MacDonald Center, 1 N. MacDonald St., Ste. 15, Mesa, AZ, United States, 85201, (888) 866-7638,, .

Katherine Hauswirth, APRN
Revised by Karl Finley

  This information is not a tool for self-diagnosis or a substitute for professional care.