Paracelsus’s rule still stands: the dose makes the poison
In science, a poison is any chemical that harms or kills a living organism. The sixteenth-century physician Paracelsus, often called toxicology’s father, put it sharper: everything is poison, and only the dose makes a thing not a poison. Even water can kill in excess.
Human poisoning symptoms can mimic other illnesses and vary by substance and organ system—altered consciousness, abnormal temperature, irregular heart rate, breathing changes. Caustics may injure mouth, throat, gut, and lung membranes with pain, cough, vomiting, and shortness of breath. Exposure routes include ingestion, inhalation, injection, or skin absorption. The English word appears around 1200 from Old French for a drink, tracing to Latin potio; "poisonous" as an adjective dates to the 1520s, poison oak to 1743, poison ivy to 1784, and poison gas to 1915. Legal "poison" labels are narrower than everyday scare language for corrosives, carcinogens, or pollutants.
Biology and medicine parse related words carefully. Toxins are toxicants made by living organisms—bacterial proteins behind tetanus or botulism, for example. Venoms are toxins delivered actively by bite or sting; poisons are toxins delivered passively when swallowed, inhaled, or absorbed. A poisonous organism harms when eaten or touched; a venomous one injects. Few organisms are both. Chemicals toxic mainly to pathogens but not humans are usually called antibiotics, as with Penicillium products. Nuclear physics even uses "nuclear poison" for neutron absorbers that slow reactors. Environmentally hazardous spills such as food-processing wastewater can starve water of oxygen without being human poisons. Fat-soluble toxins may bioaccumulate up food chains in predators even when first eaters show little harm.
Industry uses many poisonous agents for non-homicidal ends—medicines, solvents, cleaners, coatings, feedstocks—and sometimes for the toxicity itself in weed and pest control. Natural pesticides such as concentrated salt against slugs are ancient; synthetic insecticide bioaccumulation worries bird-eating species. Contact routes matter: hydrofluoric acid is a notorious skin poison; sour gas can kill quickly in air; plant irritants like poison ivy are often classed as allergens that turn immunity against the body. In 2013, 3.3 million unintentional human poisonings occurred worldwide, causing 98,000 deaths, down from 120,000 in 1990. Chelation and filtration can sometimes reverse contamination, but entropy makes cleanup costly. The thin line between drug and poison shows in agents such as fentanyl, whose median lethal dose sits only slightly above the effective dose.
Source: Poison
