Liquid Medicines and Injections

4. Setting up and solving the formula - Syrups and Suspensions

Flucloxacillin Suspension

A drug chart shows that a patient requires 375g of Flucloxacillin. The label on the bottle gives the strength as 125mg/5ml. 

How much medicine should you give to the patient?

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.

No conversion is necessary in this example.

Step 2: Set up the NHS1 formula

This patient Needs 375mg, and we Have 125mg. Our Stock number is 5ml. 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 medicine 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, we know that we are giving 3 lots of 5ml to make a 15ml dose. We need to check that 125 x 3 actually does equal 375. If it does, we can be confident that our answer is correct.