What Is Sterile Water for Injection? Uses, Dosage, and What You Need to Know in 2026

Sterile Water for Injection, USP (commonly abbreviated SWFI) is one of the most essential yet least-understood pharmaceutical products in medicine. Millions of patients receive medications every day that would not be possible to administer without it. Yet many patients don't know what it is, why it exists, or why it's been so difficult to find in recent years. This guide explains everything you need to know.

What Is Sterile Water for Injection?

Sterile Water for Injection, USP is a sterile, nonpyrogenic (fever-causing contaminant-free), preservative-free preparation of water for pharmaceutical injection use. It is chemically designated H₂O. Despite being 'just water,' it is a tightly regulated FDA-approved drug product that must meet strict standards for:

Sterility (complete absence of bacteria, fungi, and other microorganisms)

Nonpyrogenicity (absence of endotoxins and pyrogens that cause fever)

Particulate matter limits (essentially particle-free to prevent vascular complications)

pH specification of 5.0–7.0

SWFI is available only in single-dose containers (vials ranging from 2 mL to 1,000 mL bags and pharmacy bulk packages up to 3,000 mL). It is labeled 'Rx only' in the United States.

What Is Sterile Water for Injection Used For?

SWFI has one primary purpose: serving as a pharmaceutical diluent. It is used to dissolve or dilute medications that come as dry powders or concentrated liquids before they are injected into a patient. Specific uses include:

Reconstituting lyophilized (freeze-dried) drugs: Many antibiotics (cefazolin, acyclovir), vaccines, biologics, and cancer drugs are manufactured as powders for stability and reconstituted with SWFI before injection.

Diluting concentrated drug solutions for IV administration.

Preparing admixtures for hospital IV push or IV infusion delivery.

Supporting home infusion therapy for conditions requiring long-term injectable antibiotic, biologic, or hormone treatment.

How Is Sterile Water for Injection Dosed?

There is no universal 'dose' of SWFI because the volume used depends entirely on the specific drug being reconstituted. The prescribing information for each medication specifies exactly how many mL of SWFI to add. For example:

A 1 g vial of cefazolin may require 3 mL SWFI added for reconstitution.

Alteplase 2 mg vials require 1.1 mL SWFI added, as specified in the product's prescribing information.

Growth hormone products specify the exact mL of Bacteriostatic Water or SWFI needed based on vial size and desired concentration.

Always follow the exact reconstitution instructions in your medication's prescribing information or as directed by your pharmacist. Using the wrong volume changes the drug concentration and can affect safety and efficacy.

How to Store Sterile Water for Injection

SWFI should be stored at controlled room temperature: 20°C to 25°C (68°F to 77°F). Do not freeze. Single-dose vials must be discarded after opening—any unused portion should be thrown away. Once opened, a single-dose vial cannot be used again.

Bacteriostatic Water for Injection (which contains benzyl alcohol preservative) can be used for multiple doses from a single 30 mL vial—it should be discarded 28 days after first entry.

Is Sterile Water for Injection the Same as Bacteriostatic Water?

No. They are similar but distinct:

SWFI: Preservative-free, single-dose only, must be discarded after use. Required for medications that specify preservative-free diluent, and for neonates.

Bacteriostatic Water for Injection (BWFI): Contains 0.9% benzyl alcohol as a preservative. Allows multiple withdrawals from a single vial for up to 28 days. Contraindicated in neonates and small infants, and for intrathecal/epidural injections.

Who Manufactures Sterile Water for Injection?

The U.S. market is served primarily by four major manufacturers: Baxter International (approximately 60% market share), B. Braun Medical (approximately 23%), ICU Medical (which acquired Hospira's products from Pfizer), and Fresenius Kabi/APP Pharmaceuticals. This concentration makes SWFI supply extremely sensitive to manufacturing disruptions at any single facility.

Why Is Sterile Water for Injection In Shortage?

SWFI has been on the FDA's active shortage list since November 2021 due to concentrated manufacturing, low profit margins, and catastrophic weather events (Hurricane Maria in 2017, Hurricane Helene in 2024). For a full explanation, see: Why Is Sterile Water for Injection So Hard to Find?

How to Find Sterile Water for Injection In Stock

Use medfinder to check pharmacies near you for current stock. Enter your medication name, vial size, and ZIP code—medfinder calls pharmacies on your behalf and texts you the results.