Tablets and Capsules

1. General Numeracy Review

You must be secure with the following rules before beginning work on drug calculations:

• Fractions can be converted into whole or decimal numbers by dividing the top number (numerator) by the bottom number (denominator). For example, ¾ is equal to 3 ÷ 4 = 0.75
• A whole or decimal number can be converted to a fraction by dividing it by 1. For example, 3 can be represented by the fraction ³/1
• To multiply fractions, first multiply the top numbers (numerators) and write this number at the top of your new fraction. Then multiply the bottom numbers (denominators) and write this answer at the bottom of your new fraction. Then cancel down.
• When multiplying, you stay on the top of the line all the way across and then on the bottom line all the way across.
• If you are multiplying a fraction by a whole number, you need to put that whole number over 1 to create the correct fraction - e.g. a 5ml stock solution becomes 5/1. Multiplying both the numerator and the denominator by 5 would give you the wrong answer.

2. What is a drug calculation formula?

A drug calculation formula is a reliable method for setting up the medication information that you have been given. This becomes a mathematical problem which, when solved, will give you the correct dosage for your patient.

You will come across a number of different formulae, depending on the medications and delivery routes involved. You will also find that there is more than one way to set up a particular kind of calculation. Our advice on this is as follows:

FIND THE FORMULA THAT WORKS FOR YOU AND STICK WITH IT!

If you do this, then it is possible to get by with learning just 3 different formulae, providing you know and understand them well.

Reading comprehension is essential to successful drug calculations. It is vital that you are able to understand the meaning of the information given so that you can set up the formula correctly. 

The following chapter will take you through the basic NHS1 formula, with clear explanations of each element, and guidelines for setting up the formula and solving it. This is a relatively straightforward calculation when applied to tablets and capsules. However, a sound understanding of the principles involved will ensure that you are just as confident to perform it when working with liquid medicines and injections.

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.

4. Setting up and solving the formula

A drug chart shows that a patient requires 0.75g of Amoxicillin. The label on the box of capsules gives the strength as 250mg. 

How many tablets should you give to the patient?

There are several questions that you should consider at this stage:

1. What am I being asked?
2. What units am I working with?
3. Do I need to convert any units?
4. How do I set up the formula?
5. How do I solve the formula?
6. Does my answer make sense?

Step 1:  Make sure both amounts (NEED and STOCK) are in the same units

In this case, make sure that both amounts are in milligrams. If you are unsure about how to convert between units of measure, please work through the material in Topic 3 - SI Unit Conversions.

0.75 g = 750 mg (Need) 

250 mg (Stock)

Step 2: Set up the NHS1 formula

We Need 750g, we Have 250mg and our Stock is 1. Our formula is therefore:

Step 3: Solve the formula

We now have a relatively straightforward multiplication problem to cancel down and solve:

Step 4: Check your answer

Multiplying your answer by the strength of the capsules should take you back to the prescribed amount. If it doesn't, you need to go back and check carefully for any errors.

In this case 3 tablets at 250mg each = 3 x 250 = 750. 

We can therefore be confident that our answer is correct.