Tablets and Capsules
This short tutorial will take you through the basic formula and rules for setting up drug calculations. If solving the problems without a calculator is difficult, then you should work through the resources in Topic 2 - Fractions, Decimals and Percentages.
3. What information do I need?
Let's have a look at a basic formula for calculating medication. Here at the University of Essex, we teach a formula called NHS1:
A couple more illustrations that might help you to remember this formula are:
This is all very well, but what do these words and letters mean, and how do you extract them from the problem, chart or exam question?
N is for Need
Need = the quantity of medication that the patient needs. This is the quantity ordered by the prescriber and may not match the amount of medication in your stock tablets or capsules.
Clues to this element of the formula may be phrased in the following ways:
- Your patient requires...
- Amoxicillin 750mg is ordered...
- A patient is prescribed...
- 1mg prescribed....
- The prescriber requests...
- A prescription reads...
- The doctor prescribes...
These are not the only phrases that you will come across, but should help you to understand how to spot the clues that will help you recognise the first element of the formula - what the patient Needs.
H is for Have
This element of the formula describes the quantity of medication available to you in one tablet or capsule. It is also known as 'on hand'. You may be provided with a picture of a label to read, or the instruction may be written as follows:
- On hand is...
- The medication is available in...
- The strength available is...
- The label reads...
- Each tablet contains...
- The bottle contains 25mg capsules...
- In stock are tablets containing...
Once you have worked out what you Have, you can set up the first part of the formula.
S is for Stock
This part of the formula refers to the delivery system for the medication. It is sometimes referred to as 'what it's in'. For example, a liquid medication may contain 250mg of the drug (what you Have) in 5ml of liquid. In this case, the Stock number would be 5.
In the case of tablets and capsules however, the Stock number is always 1.
ALWAYS REMEMBER THE 1!
It is important always to remember to put the Stock number over 1. For tablets and capsules, this will give you a fraction of 1/1, which may appear unnecessary. However, you need to remember that the Stock numbers will be different for liquids and injections.
Multiplying the Have value by the Stock value, instead of by 1 is a very common error in written drug calculations.
In the next chapter, we will look at a standard prescription for antibiotic tablets, and work through how to set up the formula.