Dosage for solid medicines

In this section we will demonstrate how to calculate the required dosage of tablets to give to a patient.

We all play a vital role in checking the dose given to the patient, and any of us has the potential to catch an accidental error which might otherwise lead to the wrong dose. 

In this section, we will make reference to the ‘PEACE’ problem-solving method introduced in the 'Tackling number problems' section. You may wish to review this before going further.

Plan

Start by thinking through what you need to do.

• Your goal
• Your patient
• The units
• The consequences

Estimate

Approach

• WHAT - what method are you going to use
• HOW - will you go about it?
• WHEN and WHERE - is this the right time and place?

We will now look in more detail at the two main ways of calculating dosage: mental arithmetic and using a formula. Remember, for either of these approaches you must first be sure that the stock dosage is in the same units as the required dosage (the prescription).

Approach one - mental arithmetic

There are different ways you might use mental arithmetic to calculate dosage. Let’s look at two methods, based on the following example: A patient is prescribed 250mg of a drug that is available as 60mg tablets. Follow the slideshow below to see how the mental calculation is done.

If adding a half or quarter of a tablet still doesn't give the exact answer and exact dosage is required, you may need to find smaller dose tablets (e.g. 15mg) to allow you to arrive close to the required dose.

Mental arithmetic example

One approach is to keep adding the available dose (the 60mg tablets) until the required dose (250mg) is met. 240 mg (4 tablets) is close to the required amount (250 mg). 

Approach two- using a formula 

Now let's look at using a formula (sometimes called 'The Golden Formula') to calculate the dosage. Consider the formula below and then view an example showing how the formula calculation is done in the slideshow that follows.

N.B. Units required for stock and dose must be the same. 

Number of tablets to be given = Required dose/Stock dose

This is sometimes written as follows:

Number of tablets to be given  = What you want/What you've got

Formula example:

Let's now apply the formula to the 500 microgram tablets. In this case, the sum is 3000/500 = 6 tablets, which can be safely given.

Calculate- using the formula

When using the formula approach you may wish to work out the answer using pen and paper, or you may wish to use a calculator. Either of these approaches is acceptable. In fact, using both approaches together is a great way to check your answer.

Here are some practice examples. Try to answer them using whatever method you are most comfortable with.  Some of these you may be able to do using mental arithmetic and some will probably require you to use the formula.

Your patient has been prescribed 62.5mg of Captopril (orally).

The tablets available are 12.5 mg.

Calculate how many tablets you would need for each strength. 

The prescription (62.5 mg) is in the same units as the tablets (12.5mg), so you can simply apply the formula:

What you want = 62.5 mg

What you've got = 12.5 mg tablets

62.5/12.5 = 5 tablets

Evaluate

When you have completed your calculation, remember to check your work. Here's a reminder of the ways you might do this:

• repeat the calculation
• ask a colleague to check your answer
• try to calculate the answer again using a different method
• check against the recommended dose range (e.g. using the British National Formulary)
• look for unusually big or small answers.