ABSTRACT
When a dosage form is administered in an eye, ear, nose, or vascular site, it may cause a lot of discomfort to the patient if the preparations are not isotonic. Isotonicity refers to the equality of osmotic pressure exerted by the formulation and the body fluids such as plasma, blood, and tears. Whenever a solution is separated from a solvent by a “semipermeable membrane,” the solvent passes across the membrane into the solution. This is the phenomenon of osmosis, which may be defined as the passage of solvent molecules across a semipermeable membrane against the concentration gradient. Osmosis can also occur when a concentrated solution is separated from a less concentrated solution by a semipermeable membrane. The pressure differential that develops across the membrane is called “osmotic pressure.”
