Terms Used in Extemporaneous Compounding

What is extemporaneous compounding?

Extemporaneous compounding means preparing a medicine in the pharmacy, for one named patient, when the prescription is received. The word comes from the Latin ex tempore, "at the time".

It is used when:

• no suitable ready-made product exists, for example a liquid for a child who cannot swallow tablets;
• the patient needs a strength or dose that is not made commercially;
• the patient is allergic to a dye, preservative or other ingredient in the ready-made product;
• a ready-made product is out of stock;
• a strong product has to be diluted, for example a cream.

It also has limits. Extemporaneous preparations are made by hand and are not tested like factory-made medicines, so their stability is often not known. For this reason the pharmacist decides whether a preparation should be made, uses a reliable formula from an approved reference, and decides the expiry date. A pharmacy technician follows the formula exactly, under the pharmacist's supervision, and keeps clean, accurate records.

The key terms

Freshly prepared and recently prepared

Both terms are about how soon a preparation must be used after it is made. A freshly prepared product has the shortest life, because it breaks down or grows germs quickly. A recently prepared product has a longer life, but it still has a clear limit.

Textbooks and pharmacopoeias give different time limits for the two terms. The idea is the same in all of them: freshly prepared means a very short life, and recently prepared a longer but still limited one. In an examination, use the limits given in the textbook your school follows. For a product you are actually dispensing, the pharmacist sets the discard date from the reference used for that formula.

Products that often need a short life include water-based liquids with no preservative, and powders that are mixed with water at the time of dispensing, such as some antibiotic liquids. After mixing, these keep for only a short time, and the label states how long.

Potency

In dispensing, potency means how much active drug a preparation actually contains, usually written as a percentage of the amount stated on the label. A preparation can lose potency if the drug breaks down while it is stored. A figure often used in stability work is that a product should keep at least 90% of its stated strength until its expiry date.

In pharmacology the word has a second meaning. A drug is more potent if a smaller dose gives the same effect. Read the question carefully to see which meaning is intended.

Efficacy

Efficacy is whether the medicine works, that is, produces the intended effect. Manufactured medicines are tested for efficacy before they are sold. Extemporaneous preparations are not, so there is less evidence that they work. Efficacy can fall if the drug breaks down, if it is not mixed evenly, or if it is made with the wrong ingredients or quantities. This is why a tested formula must be followed exactly.

Shelf life

Shelf life is the period from the time a preparation is made until it no longer meets its quality standards: strength, appearance, safety and, for liquids, freedom from germs. This applies only when the preparation is stored as directed in a sealed, undamaged container. The end of the shelf life is the expiry date, or on a pharmacy label, the discard date.

What shortens shelf life?

Working out a discard date

Count the days from the day of preparation as day 0.

Example 1. Prepared on 5 October. Shelf life 14 days.

• 5 + 14 = 19 October

Example 2: crossing into a new month. Prepared on 20 October. Shelf life 14 days.

1. Days left in October after the 20th: 31 − 20 = 11
2. Days still to count: 14 − 11 = 3
3. The discard date is 3 November.

If you are unsure how the pharmacy counts, ask the pharmacist and use the same method every time.

Working out the strength left

Example. A suspension is labelled 250 mg in 5 mL. Its potency has fallen to 90%.

• Drug in each 5 mL = 250 × 90 ÷ 100 = 225 mg
• The patient receives 250 − 225 = 25 mg less per dose than the label says.

What goes on the label

Every extemporaneous preparation should carry:

• the name and strength of the preparation, and the patient's name;
• the date of preparation;
• the discard date, or a statement such as "Discard after (date)";
• the storage instruction, for example "Store in a refrigerator" or "Keep in a cool place";
• "Shake the bottle well before use" for suspensions;
• the directions for use, in plain words.

The pharmacist checks the label and the expiry date before the medicine is handed to the patient.