The story

Injectable medicine needed a clean solvent long before it needed clever molecules, and water for injection is the oldest item on any pharmacy bench. The standard came out of the hard lesson that water can look clear and still carry pyrogens, the burnt fragments of dead bacteria that spike a fever even when nothing alive is left. Distillation and reverse osmosis fixed that, and the modern grade is defined by what is absent: no organisms, no pyrogens, no additives. Russian, European and North American pharmacopoeias all carry their own near identical monograph for it, because every country that makes injections had to solve the same problem.