Mathematics: Drug Calcs

A Method for Solving Tablet Questions

1. Know your units
2. Draw a picture if you need to
3. To convert, think: Do I need to multiply or divide?
4. Calculate
5. Write your answer, using the correct units

Example

A patient requires 500mg of Codeine Phosphate. Tablets are available in 1g units. How many tablets do you give?

1. mg is miligrams and g is grams.  There are 1000mg in 1 g.
2. Draw a picture of a tablet if you like
3. Smaller to bigger units means I need to DIVIDE.
4. 500 ÷ 1000 = 0.5
5. ANSWER... 0.5g tablet (half of a 1g tablet)

A Method for Solving Liquid Drug Dosages

1. Know your units
2. Draw a picture if you need to
3. First part.   (Do I need to convert? - multiply or divide)
4. Calculate.
5. Second part.                                                                                                                       Use the formula    x Volume of stock  =  Volume required           or      x V      
6. Calculate
7. Write your answer, using the correct units

Example

You need to administer 2000mg, and the solution contains 5g/100mL. How many mL will you give?

1. mg is miligrams, g is grams and ml is mililitres.  There are 1000mg in 1 g.
2. Draw a picture if you like
3. First part.  I need to convert mg to g. Smaller to bigger units means I need to DIVIDE.
4. 2000 ÷ 1000 = 2    (2000mg = 2g)
5. Second part.     x V     D = 2g    S = 5g    V = 100mL

6.    x  100  =  40

7. ANSWER... 40mL

These problems require you to think about how long (time), but also how much (volume), and how fast (rate).

Drip rates (in drops per minute, dpm) get rounded to the nearest whole number, so no decimals!

A Method for Solving Drop Rate Problems

1. Know your units

2. Time. (Do I need to convert between minutes and hours?)

3. Calculate.

4. Work out the volume. (Do I need to convert? - multiply or divide)

5. Calculate

6. Use the formula  x Df                                                                                                                                                                                                         x drop set rate  =  Drops per minute

7. Calculate

8. Write your answer, using the correct units

Example

Your patient requires 120mg of paracetamol per hour. There is 10mg paracetamol per 5mL of solution. There are 15 drops per ml.

What drops per minute should you set?

1. mg is miligrams, and ml is mililitres.  There are 1000mg in 1 g. There are 60 minutes in 1 hour.

2. Time. I need to convert 1 hour to minutes.

3. 1 hour = 60 minutes. Therefore the patient requires 120mg per 60 minutes.

4. Volume. How many mL does my patient require? The question states 'There is 10mg paracetamol per 5mL of solution'

5. 120mg ÷ 10mg = 12          12 x 5mL = 60mL

6. V = 60mL    T = 60min    Df = 15 drops per mL

7.   x  15  =  15

8. ANSWER... 15 dpm

1 kg                   one kilogram

1 g                     one gram

1mg                   one milligram

1mcg or 1μg     one microgram

1kg   =   1000g

1g     =   1000mg

1mg  =   1000mcg or 1000μg

Micrograms can be written as mcg or μg

Liquids

1L        one litre

1ml      one millilitre

1L  =   1000mL

Converting smaller to bigger units? DIVIDE

g to kg?        g ÷ 1000

mg to g?       mg ÷ 1000

mcg to mg?  mcg ÷ 1000

mL to L?       mL ÷ 1000

Converting bigger to smaller units? MULTIPLY

kg to g?        kg x 1000

g to mg?       g x 1000

mg to mcg?  mg x 1000

L to mL?        L x 1000