Toxicology is the study of the toxic effects of substances on living organisms. It includes symptoms, mechanisms, treatments and detection of poisoning, especially the poisoning of people. The main criterion regarding the toxicity of a substance is the dose, i.e. the amount of exposure to the substance.
Adapted from British Society of Toxicology http://www.thebts.org/Careers/ToxicologyDisciplines/PharmaceuticalToxicologistsandDrugDevelopment.aspx