A toxin is a chemical, organic or inorganic, that is harmful to an organism. All drugs, even those taken as medicine, are perceived by the body as toxins. Therefore, the amount of a toxin is an important consideration in toxicology. Professionals called pharmacologists determine through experiments the proper dose of a drug. A correct dose of a drug means that its effects are helpful, and its side effects are not harmful. Even common drugs like acetaminophen (Tylenol) are poisonous when taken in excess.

The liver is the organ in the body that breaks down toxins. Pollutants that enter the body, called body toxins, are destroyed in this organ. Examples of body toxins include pesticides, mercury (present in some fish), and antibiotics and hormones (in meat.) Concern about these toxins lead some to change their lifestyles or try holistic diets and therapies to rid themselves of these toxins. The effectiveness of such remedies remains controversial.

Toxicology also plays a large role in ecology. Pollution is the introduction of toxins into an ecosystem. Environmental toxicologists follow the path of toxins through the food web and determine their effects; for example, on the survival rate of a species. Toxins in the environment directly affect our food and water supplies.

Finally, toxicology plays an important role in forensics, especially in forensic pathology (determining the cause of death.) A toxicology screen of the blood is routine in a post-mortem examination in order to rule out foul play, or to determine if drugs or alcohol played a role in death.