Powders are solid dosage form containing dry mixtures of finely divided drug substance(s) and excipients intended for internal or external use. Although the use of powders as a dosage form has been replaced largely by the use of tablets and capsules in modern medicine, they represent one of the oldest dosage forms and present certain advantages that have led to their continued use as pharmaceutical dosage forms.
Powders can be classified in various ways and these include
- Classification based on use
- Classification based on particle size and
- Classification based on dispensing/ by the way they are presented to the user.
1. Classification of powders based on use
Based on use, pharmaceutical powders can be classified as powders for internal use or powders for external use. These are brieﬂy described as follows:
a. Pharmaceutical powders for internal use
Pharmaceutical powders for internal use are preparations consisting of solid, loose, dry particles of varying degrees of fine particle size that contain one or more active substances, with or without excipients. Powders for internal use can be taken orally (e.g., Oral powders), administered through the nose as snuffs, or blown into a body cavity as an insufflation.
b. Pharmaceutical powders for external use
Topical powders also known as powders for cutaneous application or powders for external use are preparations consisting of solid, loose, dry particles of varying degrees of fineness. They contain one or more active substances, with or without excipients and, if necessary, appropriate coloring matter.
Powders for external use can be applied to compromised areas of the body. Highly sorptive powders should not be used for topical powders that are to be applied to oozing wounds, as a hard crust may form.
2. Classification of powders based on particle size
After preparation powders are classified according to their particle size. In order to qualify the particle size of a given powder, the USP uses the following descriptive terms:
a. Very coarse (No. 8) powder: All particles pass through a No. 8 sieve (2.38 mm) and not more than 20% pass through a No. 60 sieve.
b. Coarse (No. 20) powder: All particles pass through a No. 20 sieve (0.84 mm) and not more than 40% pass through a No. 60 sieve.
c. Moderately coarse (No. 40) powder: All particles pass through a No. 40 sieve (0.42 mm) and not more than 40 % pass through a No. 80 sieve.
d. Fine (No. 60) powder: All particles pass through a No. 60 sieve (0.25 mm) and not more than 40% pass through a No. 100 sieve.
e. Very fine (No. 80) powder: All particles pass through a No. 80 sieve (0.18 mm). There is no limit to greater fineness.
3. Classification of powders based on dispensing
Pharmaceutical powders are classified based on dispensing or by the way they are presented to the user into bulk or divided powders.
a. Bulk powders
Bulk powders refer to a mixture of all the materials (usually non-potent drugs), packed into a properly designed bulk containers, such as a tight, wide-mouthed glass or plastic bottle, and are intended for either internal or external administration. The major problem of bulk powders is the inaccuracy of dose.
The dose of bulk powders can be affected by many factors, including
- the measuring device (spoon)
- storage humidity
- degree of settling and
- patient factors.
The dose of bulk powder may vary for patients using differently sized spoons, or even those using the same spoon according to their technique. In addition, drugs present in the bulk powders are better suited, if they have a wider therapeutic window, a large dose, and pleasant taste.
Among the bulk powders available in prepackaged amounts are
- antacids (e.g., sodium bicarbonate) and laxatives (e.g., psyllium [Metamucil]), which the patient takes by mixing with water or other beverages before swallowing;
- douche powders (e.g., Massengill powder), dissolved in warm water by the patient for vaginal use;
- medicated powders for external application to the skin, usually topical anti-infectives (e.g., bacitracin zinc and polymyxin B sulfate) or antifungals (e.g., tolnaftate); and
- brewer’s yeast powder containing B-complex vitamins and other nutritional supplements.
b. Divided powders
Divided powders, or chartulae, are single doses of powdered drug mixtures individually enclosed in paper, plastic laminates, or metallic foil wrappers or packets. Chartula, which is abbreviated as chart, is the Latin word for powder paper.
The divided powder is a more accurate dosage form than bulk powder because the patient is not involved in measurement of the dose.
A number of commercially prepared premeasured products are available in folded papers or packets, including headache powders (e.g., BC powders), powdered laxatives (e.g., psyllium mucilloid, cholestyramine resin), and douche powders (e.g., Massengill powder packets).
- Allen, L and Ansel, H (2014). Ansel’s Pharmaceutical Dosage Forms and Drug Delivery Systems. Philadephia: Lippincott Williams and Wlkins.
- Dash, A., Singh, S. and Tolman, J. (2014). Pharmaceutics – Basic Principles and Application to Pharmacy Practice. USA: Academic Press.
- Ghosh, T. and Jasti, B. (2005). Theory and Practice of Contemporary Pharmaceutics. USA: CRC Press LLC.