water for injection
Water is the most common ingredient in injectable medicines, and it has to be the cleanest water imaginable. Water for injection (WFI) is highly purified water made specifically to be safe to put into a vein: it carries almost no dissolved salts or organic matter, and crucially it is free of pyrogens. It is the standard vehicle in which most parenterals are dissolved.
Ordinary purified water is clean enough to drink and even to make tablets, but it is not good enough to inject, because it can still carry trace endotoxin from bacteria. WFI is held to a far stricter pharmacopoeial standard, with tight limits on conductivity (a measure of dissolved ions), total organic carbon, and especially bacterial endotoxin. It is produced by distillation or by an equally rigorous membrane process such as reverse osmosis, which separates the water from anything that could be a pyrogen.
WFI as collected is not yet sterile — it is the high-purity, pyrogen-free starting water. It becomes a sterile product (Sterile Water for Injection) only after it is filled into containers and sterilized, or after the finished drug solution made with it is sterilized. To stay pyrogen-free, freshly made WFI is typically stored hot (above about 80°C) and kept constantly circulating, because still, cool water is exactly where bacteria love to grow.
A useful distinction: WFI is the vehicle and the quality standard, while Sterile Water for Injection is the packaged, sterilized product a clinician opens to reconstitute or dilute a drug at the bedside.
To reconstitute a freeze-dried antibiotic, a nurse draws sterile water for injection from a small ampoule and injects it into the powder vial — the WFI is the pyrogen-free vehicle that turns the cake back into an injectable solution.
Purified to a stricter standard and, above all, pyrogen-free — the default vehicle for parenterals.
The defining extra requirement of WFI over ordinary purified water is the endotoxin limit. That single demand drives the whole specialized system — distillation or reverse osmosis to make it, and hot circulating storage to keep it pyrogen-free.