Formulae in pharmacy are recipes from either the standard literature available or the directions of the prescriber. Ingredients can be listed as amounts, parts or percentages. The formula may also have the amount in a greater or smaller quantity than is written on the prescription.
Formulae in pharmacy originate from a period of time when all preparations were made specifically for an individual patient. At that time pharmaceutical companies did not exist. As a pharmacy student you are likely to have used these formulae when preparing preparations extemporaneously. These preparations may contain several active and inactive ingredients and so there are several calculations to complete. An error in any of these calculations has the potential to cause harm to the patient.
As ingredients with a formula have to be kept in fixed ratios, they form proportional sets. Comparison of numbers in the sets after calculation, to ensure that the proportions are maintained, is a valuable way of checking the formula and should overcome the potential for error.
Using proportional sets provides a structured approach to the problem and provides you with a method for checking accuracy.
Sometimes the formula for a product is expressed as parts rather than as quantities. The total amount of product will be the sum of the parts of the ingredients. From this, a formula can be produced and used to calculate the amounts of the ingredients in a required amount of product.
Example 6.4 involves solids in a formula expressed as parts.
It is necessary to differentiate carefully between the use of ‘parts’ and ‘to parts’. Compare the following two formulae:
A formula can also be expressed in percentages. Ointments and creams are the most common examples of this. The percentages of the ingredients can be used to produce the formula and the ingredients in a known amount of product can be calculated.
