PILLS - Calculation of concentrations

Calculation of concentrations

The strength of a drug in solution can be expressed a number of ways, all of which affect how a dose is calculated.

1. Weight (or Mass) per volume of solution e.g. morphine 10mg in 1ml, adrenaline 1:1000 (=1g per 1000ml) or magnesium 2mmol per ml
2. Units per volume e.g. insulin 100 units per ml
3. Percentage. This is the weight of a drug in grams per 100ml of solution e.g. sodium chloride 0.9%, dextrose 5%.

The mmol can be calculated if the molecular weight of a drug is known e.g. for sodium chloride the molecular weight is 58.5g per mole, therefore:

0.9g in 100ml or 9g in 1000ml

9 x 1000 = 154 mmol NaCL per L
58.5

This can be important when calculating daily electrolyte requirements for patients on intravenous fluids or Total Parenteral Nutrition

Table: Drug strength and quantity in solution

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