11.2 Dispensing Quantities & Days' Supply

Dispensing Quantities & Days' Supply

Days'-supply questions look trivial and are answered wrong constantly, because the arithmetic is easy but the reading is hard. The DHA item will bury the real instruction in the sig — a PRN, a taper, an "apply sparingly" — and your job is to convert it into a defensible number.

From sig to total quantity

Quantity = dose per administration × frequency per day × number of days. Know the Latin: OD once daily, BD twice daily, TDS three times daily, QID four times daily, PRN when required, NOCTE at night.

Worked example 1: "Amoxicillin 500 mg capsules — 1 cap TDS for 28 days." Quantity = 1 × 3 × 28 = 84 capsules. (In practice 28 days of amoxicillin would be unusual; the exam uses it purely as arithmetic.)

Worked example 2: "Prednisolone 5 mg tablets — 2 tabs BD for 28 days." Quantity = 2 × 2 × 28 = 112 tablets.

Days' supply: the master formula

Days' supply = quantity dispensed ÷ total daily use. If 84 tablets are dispensed at 3 per day, days' supply = 84 ÷ 3 = 28 days. You will need this in both directions: given days, find quantity; given quantity, find days.

Eye and ear drops

The standard conversion is 20 drops per mL for aqueous ophthalmic and otic preparations. Count both eyes: "1 drop into both eyes BD" is 1 × 2 eyes × 2 times = 4 drops/day.

Worked example 3: Timolol 0.5% eye drops, 5 mL bottle; sig 1 drop into both eyes twice daily.

1. Total drops: 5 mL × 20 drops/mL = 100 drops
2. Daily use: 2 eyes × 2 times = 4 drops/day
3. Days' supply: 100 ÷ 4 = 25 days

Insulin pens

A standard pen or cartridge holds 3 mL at 100 units/mL = 300 units per pen. A box of 5 pens = 1,500 units. Days' supply = total units ÷ units used per day.

Worked example 4: Patient injects 30 units of insulin glargine once daily; dispense one box of 5 pens.

1. Total units: 5 × 300 = 1,500 units
2. Days' supply: 1,500 ÷ 30 = 50 days

Watch for the priming (air-shot) trap: pens are primed with about 2 units before each injection. If the question includes priming for a once-daily injection, daily use becomes 30 + 2 = 32 units, and 1,500 ÷ 32 = 46.9 → 46 days. Only apply priming when the question states it.

Inhalers

Use the labelled number of actuations, not the canister weight.

Worked example 5: Salbutamol inhaler, 200 actuations; sig 2 puffs twice daily. Daily use = 4 puffs; days' supply = 200 ÷ 4 = 50 days. If the sig were 2 puffs four times daily, daily use = 8 puffs and the same canister lasts only 25 days.

Topical quantities and the finger-tip unit

One finger-tip unit (FTU) — cream squeezed from fingertip to the first crease of an adult index finger — weighs about 0.5 g and covers roughly two adult palm areas (about 2% of body surface area).

Worked example 6: Betamethasone cream, apply to both hands (≈ 2 FTU = 1 g per application) twice daily for 28 days.

1. Daily quantity: 1 g × 2 = 2 g/day
2. Total: 2 g × 28 = 56 g → dispense the nearest pack, one 60 g tube

Reconstituted antibiotic suspensions

Two separate skills are tested: the volume to dispense, and the beyond-use date.

Worked example 7: Amoxicillin 125 mg/5 mL suspension; sig 5 mL TDS for 10 days.

1. Daily volume: 5 × 3 = 15 mL/day
2. Total: 15 × 10 = 150 mL
3. If the product comes as a 100 mL bottle after reconstitution, dispense 2 bottles (200 mL) — you cannot dispense half a reconstituted bottle

Reconstitution itself means adding the stated volume of water (often in two portions, shaking between) to reach the marked final volume. Beyond-use date: amoxicillin suspension is typically stable for 14 days refrigerated once reconstituted — a 150 mL course is fine, but note a 20-day supply of the same suspension would breach its BUD and must be flagged.

Maximum daily dose and early-refill checks

A quantity review is incomplete without a dose check. "Paracetamol 500 mg, 2 tablets QID" = 8 × 500 mg = 4,000 mg/day = 4 g/day, the adult maximum — acceptable but not to be exceeded; "2 tablets five times daily" would breach it and requires prescriber contact.

Early-refill math: a 30-day supply dispensed on 1 March is next due on 31 March. Many payers permit refill at roughly 85–90% consumption: 90% of 30 days = day 27, so a request on 27–28 March is borderline-allowable; a request on 20 March (67%) is a genuine early refill needing scrutiny.

Error traps

• PRN directions: use the maximum permitted frequency ("1–2 tabs every 6 hours PRN" → 2 tabs × 4 = 8/day) unless told otherwise.
• Alternating doses: "1 tab on odd days, 2 tabs on even days" over 30 days = 15 × 1 + 15 × 2 = 45 tablets, not 30 × 1.5.
• Liquid rounding: round to a measurable volume — 137 mL calculated means a measurable 140 mL or the nearest pack size, never an odd unmeasurable figure.