A substance introduced into the body, and the chemistry by which it acts. The carrier is the substance, not the route: xenon (a noble-gas anesthetic with receptor-level neuroprotection interest) and methylene blue (a redox dye and mitochondrial electron-transport agent) sit here by that rule, and injectable and infusion agents belong to the same family — route of administration is a row attribute, not a family. (The individual infusion rows are held for a dedicated wave with per-agent evidence; the family boundary is settled here, the rows are not.) Split from the gas family because the mechanism is pharmacologic, and kept distinct from photonics even where — as with methylene blue's photodynamic facet — the two touch.