Solutions of drugs administered by parenteral and ophthalmic routes enter into the respective body fluids i.e. blood and tears or lachrymal fluid. Hence these solutions must be physiologically acceptable. The greatest determinant of this physiological acceptability is osmotic pressure.
Thus, a drug solution having osmotic pressure equal to that of blood will be physiologically acceptable. Similarly, a drug solution having the osmotic pressure matching with the lachrymal secretion will also be physiologically acceptable. If the properties of drug solution are not matched with the body fluid in solution, the irritation and cell destruction may occur.
Osmotic pressure and freezing point depression are the colligative properties i.e., they depend on the number of solute particles in the solution irrespective of whether these are large or small or ions or molecules.
Two solutions having the same osmotic pressure are called iso-osmotic or isotonic. Solutions which do not have the same osmotic pressure are called paratonic. Paratonic solutions can be either hypertonic or hypotonic. Hypertonic solutions contain more quantity whereas hypotonic solutions contain less quantity of solute(s) than required for making them isotonic. Generally a pharmacist receives prescriptions for isotonic solutions but rarely he may also have to deal with a paratonic prescription. Intravenous solutions should be isotonic with blood while ophthalmic solutions should be isotonic with tears, Blood serum and tears have the same osmotic pressure.
The osmotic pressure of a 1.8% solution of urea and 0.9% sodium chloride solution are same as that of body fluids including blood and lachrymal fluid. Thus 0.9% sodium chloride solution is isotonic with physiological fluids. Isotonic and iso-osmotic are not necessarily equivalent. It is possible that an iso-osmotic solution may cause hydrolysis of RBCs upon injection. This happens because the solutes diffuse through the membranes of the cells.
Adjustment of tonicity calls for the addition of some inert material compatible with drug substance. In practice the following methods are used for calculation of isotonicity.