BMS 113 · Common Core (Tronc commun)Section 6.3 · Prescription writing, dosage and safe medicine use
Section 28 of 31
Dose, dosage and calculations
Calculate the quantity of a medicine to dispense and an infusion rate from a prescription, and describe the dispensing protocol.
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Why this matters
If you confuse a dose with a dosage, or skip the proportion, you can dispense the wrong quantity even when every number on the order is correct. In practice a patient may run out of medicine before the course ends, or receive far more than was intended. You will reuse these calculations in Unit 6.6, when you check stock and shelf-life, and every later dispensing decision in this course assumes you can do them.
What you need to know first
You only need school arithmetic — multiplying, dividing and simple proportions — plus the reading skill from Unit 6.2. Unit 3.3 explains why liquid forms are labelled as milligrams per millilitre, which helps but is not required.
Words to know (6)
Dose
The amount of a medicine given at one time, such as one tablet, 5 ml or one injection. One 500 mg tablet is one dose.
Dosage (posology)
The amount of a medicine and how often it is given over a period, not just the amount at one time. Two tablets three times a day for three days.
DOT (duration of treatment)
The length of time a course of treatment lasts, written in the dose × frequency × DOT expression. In “2 tid × 3/7”, the DOT is three days.
Proportion
A statement that two ratios are equal, used to find a fourth unknown value when three are known. If one day needs 6 tablets, three days need 18.
Drop size
The volume of one drop of infusion fluid, about 0.06 ml for an aqueous solution, though it depends on the giving set. 500 ml ÷ 0.06 ml is about 8333 drops.
Dispensing
The preparation and issuing of drugs on a medical prescription, carried out by a pharmacist. Assessing, selecting, relabelling, counselling, repackaging and recording.
One idea at a timeIdea 1 / 7
In one sentence A dose is the amount given at one time, while a dosage is that amount repeated over a period.
Dose and dosage are not the same
A dose is the amount of a medicine given at one time: one tablet, 5 ml, one injection. Dosage, also called posology, is the amount and the frequency over a period — for example two tablets three times a day for three days. The two are not interchangeable. A correct dose taken at the wrong interval can fail or can poison, and the right interval with the wrong dose is equally unsafe. Posology is what the prescription encodes in dose × frequency × DOT, where DOT is the duration of treatment. The pharmacist reads the whole expression, because the quantity to dispense is derived from all three parts together, and a mistake in any one of them changes the total.
For example
“2 tablets three times a day for 3 days” is a dosage: it fixes the dose (2 tablets), the frequency (three times a day) and the period (3 days), and it yields 18 tablets in total.
Think of it like
A dose is like one step and a dosage like the whole route with its pace, but a route can be shortened at will, while a dosage often cannot: stopping early may let the illness return.
Pause and check yourself
What is the difference between a dose and a dosage?
Show the answer
A dose is one amount at one time, such as one tablet. A dosage adds the interval and the period, such as two tablets three times a day for three days.
Read detailed explanations
Why proportions, not memorised formulas
The calculations in this unit look different — tablets, syrups, vials, infusions — but they are the same proportion seen from different angles. A proportion states that two ratios are equal, and it lets you find an unknown fourth value when three are known. For a unit-dose tablet the ratio is one tablet per one dose, so the unknown is a count. For a liquid the ratio is a mass per a volume, so the unknown is a volume. For a vial the ratio is a mass per a container, so the unknown is a number of containers. Writing the ratios with their units, then cancelling the units that appear above and below a fraction, turns the arithmetic into a check on itself: if the answer does not come out in tablets, millilitres or vials, the setup is wrong before any number is calculated. A second check is to reverse the calculation: if 18 tablets were dispensed for a 2 × 3 times a day, 3-day order, then 18 ÷ 3 = 6 tablets a day and 6 ÷ 3 = 2 tablets per dose, which returns the original prescription. That reversal catches most errors. It also exposes why decimal and unit mistakes are so damaging: they do not change the shape of the proportion, only its scale, so the reasoning can feel perfectly correct while the patient receives a thousand times too much.
Drops, drop factors and infusion safety
The figure of 0.06 ml for an aqueous drop is an approximation, and a beginner should understand both why it is useful and where it stops being reliable. It is useful because it converts a volume order into a countable rate, which is how a gravity infusion is actually regulated: the nurse counts drops in a drip chamber because there is no volume scale on the tubing. It stops being reliable because the true drop size depends on the giving set, the fluid, the temperature and the height of the bag. A set with a different drop factor delivers a different volume per drop for the same count, so the same “35 drops per minute” can mean different flows through different tubing. This is why infusion pumps, which measure volume directly, are preferred when precision matters, and why the same 500 ml over 4 hours should be written as a volume and a time rather than only as a drop rate. The safety reasoning generalises: whenever a rate is assessed by counting rather than by measuring, human factors enter the measurement. The consequence of getting a rate wrong scales with the drug: too fast an infusion of a large volume can overload the circulation, and too fast a bolus of a potent drug can cause an immediate reaction that no later care can undo. The practical defences are to state the volume and the time on the prescription, to check the drop factor of the set in use, and to watch the patient, not only the chamber.
Explore the diagram
Look at the whole drawing first. Then select one numbered part and connect it to the idea you just read.
Calculating a dose: worked examples
The four-step dose calculation flow, three worked examples, an infusion drop-rate calculation and the dispensing protocol with its no-alteration rule.
New to diagrams? Read this first
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01 / 6The four-step calculation flow
If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.
The four-step calculation flow
Every dose calculation follows the same four steps: read the prescription and identify drug, strength, form, dose and duration; convert all units and measures, for example spoons to millilitres; multiply the single dose by the frequency to get the daily amount; then multiply by the duration and divide by the pack size, rounding up to whole packs.
Schematic; not to scale. The figures are those of the worked examples in the lecture notes; rounding rules and pack sizes differ between pharmacies.
Key points to remember
✓A dose is one amount at one time; a dosage is that amount repeated over a period.
✓Work out the amount for one day, then multiply by the number of days.
✓Liquids are labelled as milligrams per millilitre, so the quantity calculated is a volume, and vials are counted only after matching the dose to the container.
✓For an infusion, divide the volume by the drop size, then divide by the minutes.
✓The pharmacist follows the dispensing protocol and never alters an order without the prescriber’s consent.
Go deeper
Worked example
Calculating what to dispense, and how fast it should flow. A pharmacist receives a batch of prescriptions: paracetamol 500 mg tablets written “2 tid × 3/7”; amoxicillin 125 mg/5 ml syrup written “1 tablespoon bid × 5/7” with 100 ml bottles in stock; atropine 3 mg/2 ml vials written “3 mg od × 7/7”; and a quinine 500 ml infusion to flow over 4 hours. Work out each quantity and the drip rate.
Paracetamol: for one day the patient takes 2 × 3 = 6 tablets; for 3 days the patient takes 6 × 3 = 18 tablets. Dispense 18 tablets.
Amoxicillin: one tablespoon is 15 ml; for one day the patient takes 15 × 2 = 30 ml; for 5 days the patient takes 30 × 5 = 150 ml; 150 / 100 = 1.5 bottles, so dispense 2 bottles with instructions to discard the leftover after the 5 days.
Atropine: the prescribed 3 mg is the whole 2 ml vial, so the volume per day is 2 ml; for 7 days the volume is 2 × 7 = 14 ml; with 2 ml vials available, dispense 7 vials.
Quinine infusion: an aqueous drop is about 0.06 ml, so 500 ml ÷ 0.06 ml ≈ 8333 drops; the time is 4 × 60 = 240 minutes; 8333 ÷ 240 ≈ 35 drops per minute.
Every quantity comes from the same proportion method: work out one day or one unit, then multiply by the period. Checking the units and the container size is what keeps the arithmetic honest.
Common misconception
Beginners often think the strength (the mg figure) decides how many tablets to dispense. For unit doses it does not: the count comes from the dose, frequency and duration, while the strength is used to check that the order suits the patient.
Check your reasoning
A prescription asks for 150 ml of syrup and the pharmacy stocks 100 ml bottles. Why is it wrong to dispense one bottle and tell the patient to take a little less each day?
Reveal an answer
Because reducing the volume per dose silently changes the prescribed dose. The prescription fixes the dose and the duration; the pharmacy must supply the amount needed (two bottles) and instruct the patient to discard the remainder after day 5, not improvise a smaller dose.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Drug dosage calculations and dispensing protocol, pp. 57–58
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About this course and its sources
Bilingual lessons organised from the supplied BMS 113 General Pharmacology I lecture notes. Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.