Toxicology is a scientific discipline, overlapping with biology, chemistry, pharmacology, and medicine, that involves the study of the adverse effects of chemical substances on living organisms and the practice of diagnosing and treating exposures to toxins and toxicants. The relationship between dose and its effects on the exposed organism is of high significance in toxicology. Factors that influence chemical toxicity include the dosage, duration of exposure (whether it is acute or chronic), route of exposure, species, age, sex, and environment. Toxicologists are experts on poisons and poisoning. There is a movement for evidence-based toxicology as part of the larger movement towards evidence-based practices. Toxicology is currently contributing to the field of cancer research, since some toxins can be used as drugs for killing tumor cells. One prime example of this is ribosome-inactivating proteins, tested in the treatment of leukemia.

The word toxicology (/ˌtɒksɪˈkɒlədʒi/) is a neoclassical compound from Neo-Latin, first attested c. 1799, from the combining forms toxico- + -logy, which in turn come from the Ancient Greek words τοξικός toxikos, "poisonous", and λόγος logos, "subject matter").

The earliest treatise dedicated to the general study of plant and animal poisons, including their classification, recognition, and the treatment of their effects is the Kalpasthāna, one of the major sections of the Suśrutasaṃhitā, a Sanskrit work composed before ca. 300 CE and perhaps in part as early as the fourth century BCE. The Kalpasthāna was influential on many later Sanskrit medical works and was translated into Arabic and other languages, influencing South East Asia, the Middle East, Tibet and eventually Europe.

Dioscorides, a Greek physician in the court of the Roman emperor Nero, made an early attempt to classify plants according to their toxic and therapeutic effect. A work attributed to the 10th century author Ibn Wahshiyya called the Book on Poisons describes various toxic substances and poisonous recipes that can be made using magic. In the 12th century, Jewish physician Maimonides wrote Kitāb al-Sumūm wa-l-Mutaḥarriz min al-Adwiya al-Qattāla ("Book on Poisons and the One Who Guards Against Deadly Drugs"), which discussed the treatment of poisoning. A 14th century Kannada poetic work attributed to the Jain prince Mangarasa, Khagendra Mani Darpana, describes several poisonous plants.

The 16th-century Swiss physician Paracelsus is considered "the father" of modern toxicology, based on his rigorous (for the time) approach to understanding the effects of substances on the body. He is credited with the classic toxicology maxim, "Alle Dinge sind Gift und nichts ist ohne Gift; allein die Dosis macht, dass ein Ding kein Gift ist." which translates as, "All things are poisonous and nothing is without poison; only the dose makes a thing not poisonous." This is often condensed to: "The dose makes the poison" or in Latin "Sola dosis facit venenum".

Mathieu Orfila is also considered the modern father of toxicology, having given the subject its first formal treatment in 1813 in his Traité des poisons, also called Toxicologie générale.

In 1850, Jean Stas became the first person to successfully isolate plant poisons from human tissue. This allowed him to identify the use of nicotine as a poison in the Bocarmé murder case, providing the evidence needed to convict the Belgian Count Hippolyte Visart de Bocarmé of killing his brother-in-law.

The goal of toxicity assessment is to identify adverse effects of a substance. Adverse effects depend on two main factors: i) routes of exposure (oral, inhalation, or dermal) and ii) dose (duration and concentration of exposure). To explore dose, substances are tested in both acute and chronic models. Generally, different sets of experiments are conducted to determine whether a substance causes cancer and to examine other forms of toxicity.