PHAR2911 · Pharmaceutics And Professional Practice
Extemporaneous Compounding & Dispensing
Extemporaneous compounding is preparing a medicine for an individual patient at the time of dispensing from raw ingredients, when no suitable licensed product exists. This chapter covers the dispensing labs' core skills: when to compound, building the right excess, mixing by the method of doubling, and the recurring dispensing calculations — % w/w, % w/v, % v/v, parts, dilution (C₁V₁ = C₂V₂) and the alligation cross. These calculations and the dispensing record are graded on a strict pass/fail rubric where essentially every error is unsatisfactory, so they are high-yield exam material.
What this chapter covers
- 01When to compound: no product / dose / form / allergy
- 02The dispensing record & QA declaration
- 03Excess rules: capsules +2, beaker +10 g, suppositories +3
- 04Method of doubling (geometric dilution)
- 05Percentage strengths: % w/w vs % w/v vs % v/v
- 06Parts and dilution (C₁V₁ = C₂V₂)
- 07Alligation cross: blending two strengths to a target
- 08Suppository displacement value (DV)
Dilute a stock cream to a lower target strength
- +1Add the semisolid beaker excess of +10 g, so calculate for a total of 50 + 10 = 60 g of finished cream.
- +2Conserve drug mass with C₁V₁ = C₂V₂ (here masses, w/w): mass of 2% stock = (target % × total) ÷ stock % = (0.5 × 60) ÷ 2 = 30 ÷ 2 = 15 g of the 2% stock.
- +1Plain base = total − stock = 60 − 15 = 45 g of drug-free base.
- +1Back-check the drug content: (15 g × 2%) ÷ 60 g = 0.30 g ÷ 60 g = 0.5% w/w — correct. Mix by the method of doubling, then weigh out the prescribed 50 g into the final container.
Key terms
- Extemporaneous compounding
- Preparing a medicine for an individual patient at the time of dispensing, from raw ingredients, when no suitable licensed product exists. A licensed (GMP-manufactured) product is always preferred.
- Excess
- A small amount prepared above the prescribed quantity to cover material lost to the mortar, beaker or mould. Canon: capsules +2 units, semisolid in a beaker +10 g, suppositories +3 units (nil excess on a slab).
- Method of doubling (geometric dilution)
- A mixing technique for blending a small amount of active uniformly into a larger amount of base: combine the active with an equal amount of base, then keep doubling the amount added at each step until all the base is incorporated, giving a homogeneous product.
- % w/w, % w/v, % v/v
- Percentage strength conventions: % w/w = grams per 100 g (solid in solid/semisolid); % w/v = grams per 100 mL (solid in liquid); % v/v = mL per 100 mL (liquid in liquid). The denominator must match the physical state.
- Alligation
- A method for mixing a high-strength stock (H) and a low-strength stock (L) to a target (T) lying between them (L < T < H). Parts of LOW = |H − T|, parts of HIGH = |T − L|; the total is split in that ratio.
- Displacement value (DV)
- For suppositories, the grams of drug that occupy the same volume as 1 g of base. Base required = (theoretical base for N suppositories) − (mass of drug ÷ DV). Not needed when strength is expressed as a % of the medicament.
Extemporaneous Compounding & Dispensing FAQ
When is a pharmacist allowed to compound rather than dispense a licensed product?
Only when a clinical need cannot be met commercially: no licensed product exists for the drug, the required strength or dose is unavailable (e.g. a paediatric fraction), a different dosage form is needed (a liquid for someone who cannot swallow), or there is an allergy/intolerance to an excipient. A licensed product is preferred every time because it is GMP-manufactured and quality-controlled; you never compound just to make something cheaper or more convenient.
How do I use the alligation cross?
Confirm the target lies between the two stocks (L < T < H). Cross the diagonals: parts of the LOW stock = |H − T|, parts of the HIGH stock = |T − L|. Split the total amount in that ratio (each part = total ÷ sum of parts), then back-check that the weighted average equals the target. Treat pure drug as 100% and plain base as 0% if a stock sits at an extreme.
What is the difference between % w/w, % w/v and % v/v?
% w/w is grams per 100 g (used for solid-in-solid or semisolid, e.g. a cream), % w/v is grams per 100 mL (solid dissolved/dispersed in a liquid), and % v/v is mL per 100 mL (liquid-in-liquid). A 1% w/w cream and a 1% w/v solution are NOT interchangeable — picking the wrong denominator is a classic dispensing-calculation error.
How much excess do I prepare for each dosage form?
Capsules: make 2 extra. Semisolids in a beaker: +10 g (none if a slab is used). Suppositories: make 3 extra. Always apply the excess BEFORE calculating quantities, then weigh out the exact prescribed amount; preparing more than 20 g over a semisolid script is graded as an unsatisfactory (wasteful) excess.
Why does a suspension need a 'Shake the bottle' label but a solution does not?
A suspension disperses insoluble solid particles in a liquid; they sediment under gravity, so the patient must redisperse them to get a uniform dose — the 'Shake the bottle' ancillary label is mandatory and omitting it is a dispensing error. A true solution has the drug fully dissolved, so it stays uniform and does not get this label.
Exam move
Treat the dispensing calculations as a drill, not a read: the marks come from a reliable method, not memory. Lock in one universal routine for the two-stock problems — (1) apply the form's excess first, (2) set up dilution as C₁V₁ = C₂V₂ or use the alligation cross (LOW parts = |H − T|, HIGH parts = |T − L|), (3) split the total in that ratio, (4) ALWAYS back-check the weighted average equals the target. Treat plain base as 0% and pure drug as 100% so every blend collapses into the same template. Separately, drill the suppository displacement value (base = theoretical base − drug ÷ DV) and the three percentage conventions until matching % type to physical state is automatic. Then practise writing a clean dispensing record (formula, full calculations, method, batch/expiry, QA declaration, labels including 'Shake the bottle' for suspensions) because on the lab rubric an unlabelled or uncalculated step fails even when the physical product is perfect.
