Formulae for calculating drug dosages

Formulae for calculating drug dosages

Solid drugs

A whole tablet is always preferable to a broken tablet as the latter will have no definite and predictable dose. Always check with the pharmacy department whether a drug in tablet form can be broken to achieve the prescribed dose.

dose required=^{strength required}/_{stock strength}×volume or number of stock tabletsExample:

A patient is prescribed 30 mg amitriptyline. Amitriptyline is available in 10-mg tablets.

Calculation:

Number of tablets required=³⁰/₁₀×1=3

Liquid drugs

volume required=^{strength required}/_{stock strength}×volume of stock solution

Example:

A child is prescribed 240 mg paracetamol. Paracetamol suspension is available as a solution of 120 mg/5 ml.

Calculation:

volume of paracetamol required=²⁴⁰/₁₂₀ ×5=10 ml

Drugs measured in units

Some medications may be available in different strengths and it is essential that the planning of administration includes the selection of a suitable combination of strengths to provide the prescribed dose in the minimum amount of tablets or liquid. amount required=^{strength required}/_{stock strength}×volume of solution

Examples with calculations:

1. A patient is prescribed 100 000 units of nystatin. The stock solution or unit measurement is 100 000 units in 1 ml of solution.

volume of solution required=^{100 000 units}/_{100 000 units}=1 ml

2. A patient is prescribed 12 000 units of heparin (for subcutaneous injection). Stock ampoules contain 25 000 units in 5 ml of solution.

volume required=^{12 000}/_{25 000}×5=^{60 000}/_{25 000}=2.4 ml

Preparation of solutions

volume required=^{strength required}/_{stock strength}×volume of stock solutionExample with calculation:

A vial of amoxicillin (500 mg) is mixed with 4.6 ml water for injections to give a concentration of 100 mg/ml. What volume of the solution is required to give an injection of 125 mg?¹²⁵/₁₀₀×1=1.25 ml

Intravenous infusions

There are two main types of giving set in use and they deliver fluid at the rate of 20 or 60 drops per millilitre.

time for infusion (hours)=^{volume (ml)}/_{rate (ml/hour)}

Example with calculation:If a patient is to receive 500 ml normal saline at the rate of 100 ml per hour, how long will it take to deliver the fluid?⁵⁰⁰/₁₀₀=5 hours

Rate of drops per minute

rate=^{volume (ml)}/_{time (hours)}×60

(a) If the giving set has a flow rate of 20 drops per millilitre:rate=^{500×20 (drops/ml)}/_5×60=33 drops per minute

(b) If the giving set has a flow rate of 60 drops per millilitre:

rate=^{500×60 (drops/ml)}/_5×6=100 drops per minute