toxicity
Think of toxicity as a measure of how much harm a substance can do to a living body. Almost anything can be toxic if you get enough of it — even water or oxygen — so toxicity is really about the relationship between an amount and the damage it produces, not a fixed label that some chemicals carry and others do not.
In pharmacology, toxicity describes a drug's intrinsic capacity to injure cells, tissues, or whole organ systems. It is studied across timescales: acute toxicity from a single large exposure, and chronic toxicity that builds up over repeated dosing. The harm can be local (where the drug touches, such as a caustic injection site) or systemic (carried by the blood to distant organs like the liver, kidney, heart, or bone marrow).
The old toxicology maxim from Paracelsus — 'the dose makes the poison' — captures the central idea: the difference between a therapeutic drug and a poison is often just the size of the dose. Toxicologists quantify this with measures such as the lethal dose in 50 percent of test animals and the therapeutic index, which compare the dose that helps with the dose that harms. A drug with a narrow gap between those two needs careful monitoring.
It is honest to note that not all toxicity is dose-predictable. Some harm depends on an individual's genetics, immune system, or prior organ disease rather than on the amount taken, which is why a 'safe' dose for most people can still injure a particular patient.
Paracetamol is safe at normal doses but becomes severely hepatotoxic in overdose, when the liver's glutathione is depleted and a reactive metabolite accumulates — a textbook demonstration that the dose makes the poison.
Same molecule, different dose, opposite outcome.
Toxicity is not the same as an adverse drug reaction. Toxicity is the drug's inherent capacity to harm; an adverse reaction is an actual harmful event observed in a patient, which may or may not stem from that intrinsic toxicity.