Here are your brief notes on Isotonic Solutions

Solutions meant for the eye should be isotonic with the tear secretion to prevent irritation due to difference in the tonicity. If the concentration of the drug is not sufficient to yield an isotonic solution, addition of requisite amount of an inert substance compatible with drug is essential.

The pharmacist is required to select the substance and calculate its quantity necessary to render the solution isotonic. A given solution, from tonicity considerations, may be either isotonic or paratonic in relation to the tear secretion. If the quantity of the drug(s) prescribed is just sufficient to produce, on appropriate dilution, the same tonicity as the tear secretion, no substance should be added.

Such a solution is isotonic by itself. Some prescriptions direct that an isotonic solution of a drug be dispensed without stating the concentration of the drug in solution.

In such cases a pharmacist may calculate the quantity of the drug required to dispense the volume of the solution but in case that concentration is too high, he may have to refer either to the physician or use the quantity normally prescribed in the conditions for which the prescription is meant.

However, it is not infrequent that a pharmacist receives paratonic prescriptions. Such prescriptions may be of two types-hypertonic or hypotonic in relation to the tear secretion.

Hypertonic solutions contain more quantity than required to have the same tonicity as the tear secretion. It is not possible to render a hypertonic solution isotonic unless the quantity of the medicament is reduced to tonicity level. Reducing the quantity of the medication amounts to altering the prescription. The pharmacist may have to consult the physician in such circumstances.

However, a pharmacist adjusts the tonicity of the hypotonic solutions by adding appropriate quantity of soluble substances. The required quantities of such additive substances may be calculated by any one of the methods.

In practice, by experience pharmacist prepares tables of the medication in different concentrations and the concentration of the adjusting substance for the drug for all generally prescribed solutions.

Whereas it is desirable to make solutions isotonic with the tear secretion, it may be remembered that paratonic solutions within the range of 0.7 to 1.5% of sodium chloride or its equivalent in relation to other substances are easily tolerated by the eye.