Powders represent one of the oldest and most conventional dosage forms. The term “powder” has more than one connotation in pharmacy. It may be used to describe the physical form of a material, that is, a dry substance composed of finely divided particles that may be used as the basis of many other solid dosage forms such as tablets, capsules, etc.
Powders may also be used to describe a type of pharmaceutical preparation, that is, a medicated powder intended for internal or external use.
Powders were originally found to be a convenient mode of administering drugs derived from hard vegetables such as roots (e.g., rhubarb), barks (e.g., cinchona), and woods (e.g., charcoal). As synthetic drugs were introduced, powders were used to administer insoluble drugs such as calomel, bismuth salts, mercury, and chalk.
The rapid development of formulations containing high potent compounds has reduced the use of powders as a dosage form. Most of the powders are replaced by tablets and capsules. However, under certain circumstances, powders still have some advantages over solid dosage forms on the market.
A pharmaceutical powder is defined as a dry, solid substance, composed of finely divided drugs with or without excipients and intended for internal or external use. It is a solid substance in finely divided state typically obtained by crushing, grinding, or comminuting.
Properly prepared, powders have a uniform, small particle size (varying 10nm-1000μm) that has an elegant appearance. In general, powders are more stable than liquid dosage forms and are rapidly soluble, enabling the drug to be absorbed quickly.
Powders can be broadly classified by the way they are presented/dispensed to the user into bulk powders and divided powders. Other classifications of powders are based on particle size and based on use.
Bulk powders are dry, free-flowing preparations consisting of one or a mixture of finely powdered substances and intended for internal application (e.g., bulk powders for internal use) or external application (e.g., bulk powders for external use). They are supplied in bulk or large quantities and are used for less potent drugs.
Bulk powders are dosed with acceptable accuracy and safety using measuring devices such as the teaspoon, cup, or insufflator. This practically limits the use of orally administered bulk powders to antacids, dietary supplements, laxatives, and a few analgesics. Many bulk powders are used topically.
Divided powders, or chartulae, are powders in which the individual dose has been packed separately. The traditional packing of divided powders is in wrapped paper. Volatile or hygroscopic drugs are wrapped in aluminium foil or plastic bags.
Divided powders used for potent drugs (drugs that are effective in low concentrations), it was used before in old days but after development of other dosage forms it became obsolete. They are intended for internal use only.
Powders do not mix spontaneously; therefore, effective mixing requires a thorough understanding of the materials to be mixed, as well as the science of mixing. Effective mixing of powders poses the greatest challenge when the amount of one of the components of the mix is relatively small compared to the other components.
There are four main methods of mixing powders in small-scale operations:
When working with powders pharmacists should look out for efflorescent powders since they contain water of hydration, which may be released when the powders are triturated or are stored in an environment of low humidity. The water liberated can make the powder damp and pasty. Examples of efflorescent powders include caffeine, citric acid, codeine phosphate, ferrous sulfate, and atropine sulfate.
Hygroscopic and deliquescent powders should also be handled with care since these substances become moist because of their affinity for moisture in the air. Double wrapping is desirable for further protection. Extremely deliquescent compounds cannot be prepared satisfactorily as powders.
The following tips can be useful for pharmacists who need to do some compounding that involves powders:
Oral powders may be dispensed in doses premeasured by the pharmacist, that is, divided powders or in bulk. Traditionally, divided powders have been wrapped in materials such as bond paper and parchment. However, the pharmacist may provide greater protection from the environment by scaling individual doses in small cellophane or polyethylene envelopes.
Divided powders are dispensed in the form of individual doses and generally are dispensed in papers, properly folded. They also may be dispensed in metal foil, small heat-sealed plastic bags, or other containers.
Hygroscopic and volatile drugs can be protected best by using a waxed paper, double-wrapped with a bond paper to improve the appearance of the completed powder. Parchment and glassine papers offer limited protection for these drugs.