Powders and Granules

What are powders and granules?

A powder is a dry mixture of finely divided drugs or chemicals. Some powders are swallowed (often after mixing with water) and some are applied to the skin.

Granules are made by sticking powder particles together into larger pieces. They flow more easily than powder and make less dust. Granules can be given to the patient as a dosage form, or they can be an intermediate step in making tablets and capsules.

Types of powder

A few points make the difference between the types:

• Bulk powders are only suitable for drugs that are not potent, because the patient measures the dose with a spoon and spoon measures are not exact.
• Divided powders are the safe choice for potent drugs because each dose is already weighed.
• Dusting powders are for external use only. They are very fine, and they should not be applied to open wounds or raw skin unless they are sterile.
• Powders for reconstitution are supplied dry because the drug is not stable in water for long. Water is added when the medicine is dispensed. Once mixed, the liquid has a short life, so follow the label for how long it can be kept.
• Effervescent powders and granules contain an acid and a carbonate that react in water. They must be dissolved in water before taking and kept dry until then.

Advantages and disadvantages of powders

Advantages

• Large doses can be given, which is useful for drugs such as antacids.
• They dissolve and act quickly because the particles are small.
• Dry powders are usually more stable and last longer than liquids.

Disadvantages

• Unpleasant tastes cannot be hidden easily.
• Bulky and less convenient to carry than tablets.
• Bulk powders cannot be measured exactly.
• They take up moisture from the air, so they are not suitable for drugs that absorb water readily.

How powders are compounded

Compounding a powder follows the same order each time:

1. Weigh each ingredient accurately.
2. Reduce the size of any coarse particles by grinding in a mortar and pestle.
3. Sift through a sieve so the particles are a similar size.
4. Mix thoroughly. When one ingredient is a small quantity of a potent drug, use geometric dilution.
5. Divide into doses if divided powders are required.
6. Pack and label.

Geometric dilution

If a small quantity of a potent drug is simply tipped in with a large quantity of diluent, it does not mix evenly. Some doses would hold too much drug and others too little.

Geometric dilution solves this:

1. Place the smallest quantity (the potent drug) in the mortar.
2. Add an equal amount of diluent and mix well.
3. Add an amount of diluent equal to what is now in the mortar, and mix.
4. Keep doubling until all the diluent has been added.

Worked example: quantity for divided powders

Prepare 12 powders, each containing 300 mg of drug A and 500 mg of lactose.

1. Drug A needed: 12 × 0.3 g = 3.6 g
2. Lactose needed: 12 × 0.5 g = 6.0 g
3. Total weight of the mixture: 3.6 g + 6.0 g = 9.6 g
4. Weight of each powder: 9.6 g ÷ 12 = 0.8 g, which is 800 mg

When dividing, weigh each portion at 0.8 g rather than dividing by eye.

Special cases

• Hygroscopic and deliquescent powders absorb moisture and may become damp or even dissolve. Wrap each dose in waxed paper first, then in plain paper, or use an airtight sachet.
• Eutectic mixtures: some substances, such as camphor, menthol, thymol and phenyl salicylate, turn into a liquid or paste when ground together. To prevent this, mix each one separately with a light, inert absorbent powder (such as light magnesium carbonate or kaolin) before combining them.

Packaging and storage

• Bulk powders go in wide-mouthed glass or plastic jars with a tight closure.
• Dusting powders are packed in sifter-top containers so the powder can be shaken out evenly.
• Divided powders are wrapped in individual papers or sachets and placed in a box.
• Store everything in a cool, dry place, tightly closed, away from direct sunlight.

Labelling: "For external use only" on dusting powders; "Dissolve in water before taking" on powders and granules meant to be dissolved. Check the label for any storage instruction.

Granules and granulation

Granulation means turning fine powder into granules. It is done to:

• improve the flow of the powder, especially into the tablet machine;
• stop the ingredients separating after they have been mixed;
• reduce dust;
• make the powder easier to compress into tablets.

Wet granulation

1. Mix the powders.
2. Add a binder solution (for example, starch paste) to form a damp mass.
3. Pass the damp mass through a screen to form wet granules.
4. Dry the granules.
5. Screen again to get an even size, then add a lubricant (and a disintegrant, if needed).

Wet granulation uses liquid and heat, so it is only suitable for drugs that are stable to both.

Dry granulation

1. Mix the powders.
2. Compress the powder into large, hard pieces (called slugs) or sheets using a heavy press or a roller compactor.
3. Break or mill the pieces into granules.
4. Screen to the right size.

No liquid and no heat are used, so dry granulation suits drugs that are sensitive to moisture or heat.