Water for Injection
Water for Injection (WFI)
Generic Name
Water for Injection (WFI)
Mechanism
- Physical solvent: WFI provides a medium for the dissolution or suspension of active drug substances, ensuring uniform concentration and bioavailability.
- Dilution for parenteral routes: By diluting medications, it reduces local irritation and facilitates accurate dosing.
- No ion or osmolar contribution: Its isotonicity (≈273 mOsm/kg) is a characteristic of pure water, avoiding osmotic shifts when used in small volumes.
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Pharmacokinetics
- Absorption/Distribution: Not applicable—WFI acts locally at the injection site or as an IV fluid, where it rapidly distributes into the extracellular fluid compartment.
- Metabolism/Elimination: Water is eliminated primarily through renal excretion and insensible loss. When infused, the added water volume is integrated into total body water and cleared by the kidneys.
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Indications
- Diluent for injectable drugs (e.g., formulation of vaccines, antibiotics, chemotherapeutics).
- Reconstitution medium for lyophilized drugs requiring an aqueous environment.
- IV hydration adjunct in protocols that necessitate sterile water, though usually only in very small volumes.
- Preparation of oral solutions from liquid tablets or extemporaneous compounding.
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Contraindications
- Absolute contraindication in high‑volume IV therapy for patients with decompensated heart failure, cirrhosis, or severe renal impairment; large volumes of WFI can precipitate water intoxication and hyponatremia.
- Avoid in patients with hypersensitivity to residual phenols or endotoxins—WFI must be USP compliant.
- Do not use as a standalone IV fluid for hydration; preferrably isotonic saline or balanced solutions.
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Dosing
- Reconstitution: Follow the manufacturer’s instructions; typically 1–10 mL of WFI per 0.5–5 mg of drug.
- IV Use (rare): ≤ 5 mL/kg (≈ 250 mL for a 50 kg adult) when necessary; monitor serum sodium and volume status.
- Intramuscular/SC: Use small volumes (≤ 0.5 mL/dose) unless the drug formulation specifically requires larger volumes.
| Route | Typical Volume | Notes |
| IV | 1–5 mL (reconstitution) | Not for routine hydration |
| IM/SC | ≤ 0.5 mL/dose | Dilits local irritation |
| Oral | 0.5–5 mL (for compounding) | May be used as excipient |
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Adverse Effects
- Local: Mild irritation or phlebitis when injected rapidly or in large volumes.
- Systemic (rare):
- Water intoxication (hyponatremia, cerebral edema) when volumes exceed the kidneys' excretory capacity.
- Allergic reaction: Anaphylaxis extremely rare; monitor at first exposure.
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Monitoring
- Serum electrolytes (Na⁺, K⁺) if IV volumes > 250 mL or in high-risk patients.
- Vital signs (BP, pulse) during rapid IV administration.
- Urine output for patients on large-volume hydration.
- Signs of local irritation (pain, erythema).
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Clinical Pearls
- Always confirm USP sterility before using WFI, especially for pediatric or immunocompromised populations.
- Do not mix WFI with medications containing electrolytes (e.g., multi‑component antibiotic solutions) unless the formulation specifies such compatibility.
- Use “water for injection” over distilled water for parenteral preparations; distilled water may contain trace organic contaminants.
- When reconstituting high‑potency drugs (e.g., monoclonal antibodies), use the smallest volume of WFI necessary to avoid unnecessary dilution and potential inadvertent loss of drug.
- In emergency IV fluid resuscitation, request an isotonic saline or balanced crystalloid; reserve WFI for when a sterile solvent is truly required for drug reconstitution.
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• References
1. USP Good Manufacturing Practice for sterile preparations.
2. AAPS Journal: “Water for Injection: Quality, Safety, and Clinical Use”.
3. SmPCs of commonly reconstituted injectable drugs.
*This drug card is intended for educational purposes. Always consult product labeling and institutional protocols before use.*