Medication - Calculating Dosage

Calculating the correct dosage for medication administration.

1. Medications in Liquid Form

This is the formula which is commonly used to calculate the amount of medication in solution for oral, intamuscular, intravenous or subcutaneous injection to be administered, when given a dosage, stock dose and volume:

required dose ---------------------- stock dose

x

stock volume ---------------------- 1

=  volume to be given

* Note that the units for the required dose and the units for the stock dose must be the same.

Example 1 - A patient is prescribed 15mg of Stemetil. You have 2ml of solution in stock, which contains 25 mg Stemetil. What volume of solution would you give?

15 mg ------------- 25 mg

x

2 mg --------- 1 ml

=

30 ------ 25

= 1.2 ml

Example 2 - A dose of 75 mg of Pethidine has been prescribed. It is available in ampoules containing 100 mg in 2 ml. What volume must be administered?

75 mg ------------- 100 mg

x

2 ml ------ 1 ml

=

150 --------- 100

= 1.5 ml

2. Medications in Tablet Form

This formula is used to calculate the number of tablets (whole or part) to be administered when given the required dose.

required dose ------------------ stock dose

=   number of tablets to be given

* Note that the units for the required dose and the units for the stock dose must be the same.

Example 1 - A patient is prescribed 200mg of Lithium. 400 mg Lithium tablets are available. How many tablets should be given?

200 mg ------------------ 400 mg

= 1/2 tablet

Example 2 - How many tablets containing 62.5 mcg will be required to give a dose of 0.125 mg?

Step 1 : Convert to same units - convert 0.125 mg to mcg..

The decimal place is moved 3 places to the right.

0.125 mg = (0.125 * 1000 mcg) = 125 mcg

Step 2: Calculate the quantity of tablets to be given..

125 mcg ------------------ 62.5 mcg

= 2 tablets