Compounding in Pharmacy Practice
On this page
Direct answer
Compounding is the preparation of a medicine for an individual patient from raw drugs and excipients — the pharmacist's oldest craft, now confined to extemporaneous work in hospital and community pharmacies and the practical hall. Its method rests on fixed operations: correct arithmetic (percentage strengths, alligation, ratio strengths), geometric dilution to blend a small quantity of potent drug uniformly through a large bulk, trituration and levigation to reduce and wet powders, and the eutectic problem — menthol, thymol and camphor liquefying on mixing, needing adsorbent intervention. Every preparation finishes with labelling (including shake-before-use and beyond-use dating), container choice and documentation in the compounding record, and the discipline is examined through the working formula as much as the exhibit.
What you must remember
- Geometric dilution: mix the potent drug with an equal quantity of diluent, then double the bulk each trituration — one plus one, two plus two — until uniform; the only safe way to blend microgram doses.
- Percentage forms: w/w is grams per 100 g, w/v grams per 100 mL, v/v millilitres per 100 mL; ratio strength 1 in X means 1 g (or 1 mL) in X of final product — 1 per cent w/v equals 1 g in 100 mL.
- Alligation: mixing two known strengths to a desired middle strength — diagonals give the parts of each; alligation medial averages a known mixture.
- Eutectic mixtures: menthol, thymol, camphor, phenol, aspirin and chloral hydrate depress each other's melting points and liquefy on trituration — managed by triturating each separately with kaolin, magnesium carbonate or lactose before blending.
- Powder operations: trituration (dry grinding and mixing), levigation (wet grinding with a non-solvent such as glycerol for ointment incorporation), spatulation, sifting.
- Sterile compounding: aseptic technique within laminar airflow for injections and ophthalmics, a discipline of its own.
- Labelling and dating: auxiliary labels (shake well, for external use, refrigerate) and conservative beyond-use dates — aqueous preparations carry the shortest lives.
- Documentation: formula, ingredient lot numbers, double-checked calculations, preparer identity and final yield — the record that reconstructs the batch later.
An exhibit prescription worked honestly
The prescription reads: prepare 30 g of 2 per cent salicylic acid ointment, and dispense a powder containing 10 mg of a potent drug per dose — the practical hall's two archetypes. First the arithmetic: 2 per cent w/w means 0.6 g of salicylic acid in 30 g, levigated with a few drops of the base to a smooth paste before incorporation — levigation, not trituration, because the aim is wetting and deagglomeration. The potent powder is geometric dilution on display: 10 mg of drug with 10 mg of lactose triturated thoroughly, then 20 mg, then 40, doubling until the bulk carries the dose uniformly. Had the prescription combined camphor with menthol, the triturate would liquefy on the first strokes — the correct move is separate trituration of each with an adsorbent, then blending. The label carries strength, "for external use" and a short beyond-use date; the record retains the checked calculation. Marks in this subject live in the method sequence, and examiners read it off the exhibit itself.
Where compounding answers collapse
The recurring failures are arithmetic and order. Percentage questions fall apart when candidates mix w/w with w/v — 2 per cent of 30 g is a mass, 2 per cent of 30 mL a different quantity — so naming the percentage type before calculating is the discipline examiners seek. In geometric dilution, adding all the lactose at once is the standard wrong method: the point is serial doubling — uniformity at each step, since no blending is perfect. The eutectic question is answered by prevention, not repair — once liquefied the mixture cannot simply be re-powdered. Finally, beyond-use dates distinguish the safe practitioner: water-containing extemporaneous mixes carry days to weeks, not the manufacturer's years, because no stability study stands behind them; an undated or generously dated label marks an untrained compounder.
Frequently asked questions
Why is geometric dilution used for potent drugs?
Blending a few milligrams into a large bulk directly leaves distribution to chance; serial doubling homogenises at every step, making the dose uniform through the final mixture.
What is a eutectic mixture and how is it handled?
Substances such as menthol, thymol and camphor depress each other's melting points and liquefy when triturated together; each is triturated separately with an adsorbent like kaolin or lactose before the powders combine.
How does alligation solve mixture problems?
Place high and low strengths on the left, the desired strength between, subtract diagonally for parts of each — mixing two products to a target concentration.
Distinguish trituration from levigation.
Trituration is dry grinding to reduce and intimately mix powders, while levigation grinds a powder with a little non-solvent liquid into a smooth paste for incorporation into a base.
Why do extemporaneous preparations carry short beyond-use dates?
They are made without stability studies, often with aqueous vehicles inviting microbial growth and hydrolysis, so conservative dating of days to weeks is the only defensible margin.