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.
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:
- What am I being asked?
- What units am I working with?
- Do I need to convert any units?
- How do I set up the formula?
- How do I solve the formula?
- 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.