Drug calculations

Dose calculation is usually based on either body surface area (mg/m²) or body weight (mg/kg) of the patient. Body weight is used more frequently for ease of calculations.

The calculation of body surface area (BSA) used to require both weight and height.

To calculate drug doses, use the following formula:

Dose required/ Present Standard Quantity of Drug X Present Quantity of Liquid in which Standard Quantity of Drug is Dissolved

In other words:

What you want/What you have X What it is in (dilution)

For example: a patient (child) is prescribed 90mg of Paracetamol and the medication supplied is 120mg of Paracetamol in 5mls:

90 / 120 X 5 = 3.75mls

Medication errors arising from poor mathematical skills of nurses are an ongoing problem

To enhance safety:

Take time working out calculations

Recheck answers

Do not be rushed by colleagues/patients/parents/ carers

Answers that look wrong probably are wrong and an initial mental estimate of the dose may be useful.

Remember: 1 cc is the exact same amount as 1 ml!

Do YOU know the answers to the following?

1 gram = ______ milligrams (mg)

0.001 gram = _____ milligrams (mg)

1 kilogram = _____ grams (g)

0.001 kilogram (kg) = _____ gram (g)

1 liter (L) = _____ milliliters (ml) 0.001 liter (L) = _____ milliliters (ml)

1 milliliter (ml) = _____ cubic centimeter (cc)

1ml = ____ minims

4-5 ml = _____ dram

30 ml = _____ ounce

500 ml = _____ pint

1000 ml = _____ L = _____ quart

60 mg = _____ grain

1 kg = _____ pounds

Forms of medicines

Routes of administration

There are various routes of administration available, each of which has associated advantages and disadvantages. All the routes of drug administration need to be understood in terms of their implications for the effectiveness of the drug therapy and the patient’s experience of drug treatment.

Routes of administration

External, and internal (enteral, parenteral).

External: via skin, mucous, respiratory tructus.

Skin: emulsions, ligaments, decoctions, powders, mixes.

Mucous: drops, ligaments.

Nose: powders, drops, ligaments, solutions, fallow.

Ears: drops used with medicine dropper, oil solutions should been warm.

Intravaginal: drags, tampons, powders, solutions for the syringing.

Topical administration

The topical application of medicines has obvious advantages in the management of localised disease. The drug can be made available almost directly at the intended site of action, and because the systemic circulation is not reached in great concentration, the risk of systemic side-effects is reduced. For example:

The use of eye drops containing beta blockers in the treatment of glaucoma;

The application of topical steroids in the management of dermatitis;

The use of inhaled bronchodilators in the treatment of asthma;

The insertion of pessaries containing clotrimazole in the treatment of vaginal candidiasis.

Topical administration has also become a popular way of introducing drugs into the systemic circulation through the skin. The development of transdermal patches that contain drugs began with the introduction of a hyoscine-based product for the treatment of nausea in the early 1980s.

The market for such products has since grown to include a wide range of disease management areas including the prophylaxis of angina (glyceryl trinitrate), the treatment of chronic pain (fentanyl) and hormone replacement (oestrogens). While the use of transdermal drug administration is not without its problems - for example, some preparations can cause local skin reactions - many patients find it a welcome alternative to taking tablets.