---

Drug Concentrations Principles

The way the strength of a drug in a solution is described will affect the way a dose calculation is carried out. Doses may be expressed in a number of different ways:
1. Mass (weight) per volume of solution, e.g. mg in 10ml, mMol/L.
2. Units of activity per volume of solution, e.g. units per ml.
3. Percentage. This is the weight of the drug in grams that is contained in each 100ml of the solution. Common examples are 0.9% sodium chloride; 5% glucose.

% strength of solution	1ml of solution contains	100ml of solution contains	250ml of solution contains	1000ml of solution contains
1%	10mg of drug	1g of drug	2.5g of drug	10g of drug
10%	100mg of drug	10g of drug	25g of drug	100g of drug
20%	200mg of drug	20g of drug	50g of drug	200g of drug

• If you know the number of grams in 1000ml, divide by 10 to convert to % strength
• If you know the % strength, multiply by 10 to give the number of grams of drug in 1000ml
• If you know the % strength, divide by 100 to calculate the amount of drug in 1ml

Example - Calculating concentrations

The prescribed dose is 1g magnesium sulphate. Magnesium sulphate 50% injection is available. 50% = 50g in 100ml.
How many ml contains 1g magnesium sulphate?
Number of ml containing 1g = (100ml x1g) / 50g = 2ml
Another way of thinking about this is: 50g in 100ml = 5g in 10ml = 1g in 2ml