Vehicles
The medium in which the drug is dissolved or suspended.
Common vehicles include:
Water for Injection (WFI): Highly purified water used as a solvent.
Saline Solutions: 0.9% Sodium Chloride, isotonic to body fluids.
Glucose Solutions: Used for energy and as a solvent.
Lactated Ringer’s Solution: Contains electrolytes and is used for fluid replacement.
Additives
Substances added to enhance the stability, solubility, or administration of the drug, such as:
Preservatives: To prevent microbial growth (e.g., benzyl alcohol).
Buffers: To maintain pH (e.g., phosphate buffers).
Stabilizers: To prevent degradation (e.g., antioxidants like ascorbic acid).
Surfactants: To improve solubility or reduce surface tension.
Importance of Isotonicity
Isotonicity ensures that the parenteral solution has the same osmotic pressure as bodily fluids, preventing:
Cell Lysis: From hypotonic solutions causing water influx.
Cell Shrinkage (Crenation): From hypertonic solutions causing water efflux.
Maintaining isotonicity is crucial to avoid tissue irritation, pain at the injection site, and systemic complications such as hemolysis or edema.