How does the dose of a chemical determine whether it acts as a medicine or a poison?

The distinction between a medicine and a poison is fundamentally determined by the dose-response relationship, as any chemical can act as a poison depending on the amount administered. A substance is considered a medicine when it is used at a concentration that produces a desired therapeutic effect, whereas it becomes a poison when the concentration reaches levels that cause adverse or lethal effects.12

To quantify this margin of safety, scientists use the therapeutic index, defined as the ratio of the median lethal dose ($L{D}_{50}$) to the median effective dose ($E{D}_{50}$). A larger index indicates a safer drug, as there is a wider gap between the therapeutic and toxic doses. Conversely, a narrow margin increases the risk that a dose intended for treatment may prove toxic, particularly if the therapeutic and lethal dose-response curves overlap. Ultimately, while toxicity ratings help assess chemical risks, the actual hazard depends on the specific conditions of exposure and the individual's biological response.34251