University of wolverhampton
Note : The exercises provided on this sheet are for practice and so may not always
represent the actual drugs or quantities which are administered in the Trust.
A simple formula which can be used for drug dosage is:
More technically stated
A patient is ordered 1 gram of Neomycin Sulphate. 500 milligram tablets are available. How
many tablets will you give?
(When calculating for tablets treat the stock volume as 1).
Prescribed amount = 1 gram = 1000 mg
Pethidine 175mg is to be given by Intra Muscular injection. Stock ampoules of pethidine
contain 100mg in 2ml. What volume is to be drawn up for injection?
Substitute the numbers in the formula: Quantity given
= 7 = 3.5ml
TIPS FOR SAFE PRACTICE:
i) Prescriptions need to be read very carefully and different elements sorted out before starting
ii) The best way to avoid serious errors is to know what a sensible amount of the specific drug
iii) Always question unusual answers and be very careful when moving to another area or when
dealing with preparations with which you are not familiar
The following guide may be helpful in deciding whether your answer is reasonable or not.
The maximum you would normally give a patient: Tablets:
Not more than 4 for any one dose ( * with a few exceptions) Liquids
Anything from 5 ml to 20 ml for any one dose is normal Injections:
Anything from 1 ml to 10 ml for any one dose
Any answer outside these ranges probably means that you have
calculated the wrong answer
Before entering numbers in the formula, convert everything to the same units
i.e.: Always loose the decimal point and work in the smaller units
Change decimal fraction of a gram to milligrams (mg) (e.g. 0.25g = 250mg) Change decimal fractions of milligrams to
micrograms (mcg) (e.g. 0.5mg =500mcg)
PRACTICE CALCULATIONS FOR TABLET ADMINISTRATION
1. A patient is prescribed 0.3mg Thyroxine. In the ward are 100mcg tablets.
2. 750mg of penicillin is required. On hand are tablets of strength 250mg.
3. A 50 mg dose of pethidine to be given orally from stock containing 25 mg tablets.
4. Warfarin 4mg is prescribed. Stock Dose available is 1mg per tablet
5. A client is ordered 0.5milligrams of Digoxin. 250 microgram tablets are available.
How many tablets will you give? (Hint change the ordered dose to micrograms)
PRACTICE CALCULATIONS WITH ORAL MIXTURES
1. A child requires 40micrograms of paediatric Digoxin. Stock Dose is 50mcg per ml
2. A patient is to be given 750mg of penicillin, orally.
Stock mixture on hand has a strength of 250mg/5ml. Calculate the volume required.
3. 5 millilitres of syrup contains 500 micrograms of codeine.
How much syrup is given if the prescription is for 0.5 mg of the medication?
4. A patient is ordered 300mg of Sodium valporate orally.
A syrup containing 200mg in 5ml is available.
How much of the oral mixture should be administered?
5. A patient is ordered 4 milligrams of Trifluoperazine orally.
5 mg in 5 ml of liquid forte is available. How many millilitres will you administer?
PRACTICE CALCULATIONS FOR INJECTION
1. A patient is to receive 40 international units of insulin by subcutaneous injection.
The stock does is 100 international units per ml.
What volume would be drawn up into the syringe for injection.
2. An adult is to have 10mg of Cortisone.
Calculate the volume required if the stock ampoule contains 25mg in 10ml.
3. A patient is prescribed 45mg Flupenthixol Decanoate to be given by intramuscular injection.
Stock ampoules contain 30mg/ml. What volume is drawn up for injection?
4. Prescribed is 0.75mg of Brufen from stock of 0.5mg in 2 mls of solution.
What volume should be drawn to administer by injection?
5. Methicillin is stored in 250 mg in 2ml quantities. The prescription is for 62.5mg.
Volume required = Prescribed Dose x Stock Volume
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