🧠 The theory: dose, frequency, duration
Every course is really three separate numbers multiplied together: how much per dose, how many doses a day, and how many days. A dosage unit (a tablet or capsule) is just the dose expressed in whole units. Keeping these three ideas apart is what stops you multiplying the wrong pair. It is the same principle whether the medicine is a tablet, a patch or a puff of an inhaler.
units needed = (dose / strength of one unit) x doses per day x number of days
Dosing abbreviations you must know
- •OD = once a day; BD = twice a day; TDS = three times a day; QDS = four times a day
- •OM = in the morning; ON or nocte = at night; mane = morning
- •PRN = when required; STAT = immediately, as a single dose
Number of dosage units
Worked example
Amoxicillin 500 mg three times a day for 7 days. Stock is 250 mg capsules. How many capsules are needed?
- Step 1 — Work out capsules per dose. Each capsule is 250 mg and the dose is 500 mg, so 500 / 250 = 2 capsules.
- Step 2 — Scale up to one day. Three times a day means 2 x 3 = 6 capsules a day.
- Step 3 — Scale up to the whole course of 7 days: 6 x 7 = 42.
Answer: 42 capsules
Worked example
A patient needs a 1 g dose. The tablets are 250 mg. How many tablets make one dose?
- Step 1 — Put both values in the same unit. 1 g is 1000 mg.
- Step 2 — Divide the dose by the tablet strength: 1000 / 250 = 4.
Answer: 4 tablets
Tip: Convert to a common unit before dividing, or the numbers will not line up.
Worked example
Methotrexate 7.5 mg once a week for 4 weeks. Tablets are 2.5 mg. How many tablets in total?
- Step 1 — Tablets per dose: 7.5 / 2.5 = 3 tablets.
- Step 2 — This is a weekly dose, so multiply by the number of weeks: 3 x 4 = 12.
Answer: 12 tablets
Tip: Methotrexate is a weekly dose. Daily dosing is a serious, sometimes fatal, error.
Weight-based (mg/kg) dosing
Many doses are prescribed per kilogram of body weight. Multiply the dose per kg by the patient's weight to get the dose, then continue as normal.
Worked example
A child weighing 18 kg is prescribed paracetamol at 15 mg/kg per dose. What is the dose?
- Step 1 — The dose is 15 mg for every kilogram, and the child is 18 kg, so multiply the two together.
- Step 2 — 15 x 18 = 270.
Answer: 270 mg
Worked example
Gentamicin 5 mg/kg once daily for a 75 kg patient. What is the dose?
- Step 1 — Multiply the dose per kilogram by the weight: 5 x 75.
- Step 2 — 5 x 75 = 375.
Answer: 375 mg
Divided doses
A daily amount is often split across several doses. Work out the total daily dose first, then divide by the number of doses.
Worked example
A 24 kg child is prescribed 30 mg/kg/day in three divided doses. What is each dose?
- Step 1 — Find the total daily dose from the weight: 30 x 24 = 720 mg a day.
- Step 2 — Split that across the three doses: 720 / 3 = 240.
Answer: 240 mg per dose
Harder: maximum dose caps
A weight-based dose must never exceed the stated maximum, or the adult dose in a child. Always calculate, then check against any maximum given.
Worked example
A drug is dosed at 10 mg/kg, maximum 300 mg per dose. What dose suits a 35 kg child?
- Step 1 — Calculate the weight-based dose: 10 x 35 = 350 mg.
- Step 2 — Compare with the maximum. 350 mg is above the 300 mg cap.
- Step 3 — When the calculation beats the maximum, the maximum wins, so give 300 mg.
Answer: 300 mg (the maximum)
Harder: reducing-dose courses
Tapering regimens, such as a course of steroids, ask for the total number of tablets across the whole course. Work out the tablets per day at each step, multiply by the days at that step, and add them up.
Worked example
Prednisolone 5 mg tablets: 40 mg daily for 5 days, then reducing by 10 mg every 5 days until stopped. How many tablets in total?
- Step 1 — List the daily doses: 40, 30, 20 and 10 mg, each for 5 days.
- Step 2 — Turn each into tablets (5 mg each): 8, 6, 4 and 2 tablets a day.
- Step 3 — Add the daily tablets: 8 + 6 + 4 + 2 = 20 tablets across the steps.
- Step 4 — Each step lasts 5 days, so multiply: 20 x 5 = 100.
Answer: 100 tablets
💡 Exam tip
Underline the dose, the frequency and the duration in the question before you start. Most errors here come from multiplying the wrong two numbers, not from the arithmetic itself.
Test yourself
Flucloxacillin 250 mg four times a day for 5 days, using 250 mg capsules. How many capsules?
Answer: 1 per dose x 4 a day = 4; over 5 days, 4 x 5 = 20 capsules
A 30 kg child needs 15 mg/kg per dose. What is the dose?
Answer: 15 x 30 = 450 mg
A 16 kg child needs 40 mg/kg/day in four divided doses. What is each dose?
Answer: Daily = 40 x 16 = 640 mg; per dose = 640 / 4 = 160 mg
Ferrous sulfate 200 mg three times a day for 28 days, using 200 mg tablets. How many tablets?
Answer: 1 per dose x 3 a day = 3; 3 x 28 = 84 tablets