Liquid Medicines and Injections

5. Setting up and solving the formula - Injections

A patient is to receive 80mg of pethidine. The vial contains 100mg in 2ml. 

How much do you give?

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 80mg, and we Have 100mg. Our Stock number is 2ml. 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

We can use an alternative calculation strategy to check this answer. This strategy involves using a syringe as a number line, and using either repeated halving and doubling, or adding incrementally: