Powders: Definition, classification, advantages, and disadvantages
Powders: An Introduction
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.
Advantages of Powders
- There is a wide choice of ingredients, and the dose can easily be achieved for patient administration.
- Powders have better physicochemical stability and longer shelf life compared to liquid dosage forms. For example, the shelf life of powders for antibiotic syrups is 2 to 3 years, but once reconstituted with water it is 1 to 2 weeks.
- Children and adults who have trouble swallowing tablets or capsules may find powders more acceptable.
- A large dose that cannot be administered in other forms can be administered as powder. For example, if the dose of a drug is 1 to 5 g it is sometimes not feasible to manufacture tablets to supply the drug to the patient.
- Rapid dispersion of drugs occurs in the stomach when given in powder forms rather than in compressed form.
- The dissolution rate of oral powders containing water-soluble drugs is generally faster than tablets or capsules, in which disintegration of the tablet or the capsule shell is required prior to dissolution.
Disadvantages of powders
- Powders are not the dosage form of choice for drugs with an unpleasant taste. This is because masking unpleasant tastes may be a problem with this type of preparation.
- Drugs that deteriorate rapidly with exposure to the atmosphere or acidic pH should not be dispensed as powders. For example, ferrous iron salts are easily oxidized and should not be administered as powders
- Powders are bulky and inconvenient to carry.
- Powders are not a suitable dosage form for the administration of drugs that are inactivated in the stomach or drugs which can cause damage to the stomach.
- Dispensing potent drugs requiring low doses as powders (e.g., bulk powders) may not be appropriate. This is because individual doses are usually extracted from the bulk using a 5 ml spoon, which is subject to variation in spoon fill (e.g., level or heaped spoonfuls).
- Powders are not well suited for dispensing hygroscopic or deliquescent drugs.
Applications of Powders
Powders have qualities that make them an attractive dosage form for certain situations. Unlike a standardized capsule or tablet powders enable a primary care provider to easily alter the quantity of medication for each dose. Powders can also aid in clinical studies of drug preparations because the dose can be so readily adjusted. Doses can be individually weighed and placed in powder papers, envelopes, or small vials/bottles (“Powder in a bottle” research studies are an example). In another example, infants and young children who cannot swallow tablets or capsules will accept powders that can be mixed with formula or sprinkled in apple- sauce or some other appropriate food.
Also, if a drug is too bulky to be prepared as a capsule or tablet, it may be suitable for a powder dosage form. Powders provide a rapid onset of action because they are readily dispersed, have a large surface area, and usually require only dissolution, not disintegration, before absorption.
Although the use of medicated powders per se in therapeutics is limited, the use of powdered substances in the preparation of other dosage forms is extensive. For example, powdered drugs may be blended with powdered fillers and other pharmaceutical ingredients to fabricate solid dosage forms as tablets and capsules; they may be dissolved or suspended in solvents or liquid vehicles to make various liquid dosage forms, or they may be incorporated into semisolid bases in the preparation of medicated ointments and creams
Types of Powders
Among the bulk powders available in pre-packaged amounts are (a) antacids (e.g., sodium bicarbonate) and laxatives (e.g., psyllium [Metamucil]), which the patient takes by mixing with water or another beverage before swallowing; (b) douche powders (e.g., Massengill powder), dissolved in warm water by the patient for vaginal use; (c) 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 (d) brewer’s yeast powder containing B-complex vitamins and other nutritional supplements. In some cases, a small measuring scoop, spoon, or other device is dispensed with the powder for measuring the dose of the drug.
Dispensing powder medication in bulk quantities is limited to nonpotent substances. Powders containing substances that should be administered in controlled dosage are supplied to the patient in divided amounts in folded papers or packets. Patients should be educated about appropriate handling, storage, measurement, and preparation of bulk powder prescription and nonprescription products in addition to the customary counseling at the time of dispensing or purchase. Generally, these products are stored at room temperature in a clean, dry place. These products should be kept out of the reach of children and animals. Patients should be instructed how to measure the appropriate amount of the powder and be told the type of liquid or vehicle to use to deliver the medication consistent with the package and/or physician instructions.
After a powder has been properly blended (using the geometric dilution method for potent substances), it may be divided into individual dosing units based on the amount to be taken or used at a single time. Each divided portion of powder may be placed on a small piece of paper (Latin chartula; abbrev. chart.; powder paper) that is folded to enclose the medication. 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).
Divided powders may be prepared by the pharmacist as follows. Depending on the potency of the drug substance, the pharmacist decides whether to weigh each portion of powder separately before enfolding it in a paper or to approximate each portion by using the block-and-divide method. By the latter method, used only for non potent drugs, the pharmacist places the entire amount of the prepared powder on a flat surface such as a porcelain or glass plate, pill tile, or large sheet of paper and, with a large spatula, forms a rectangular or square block of the powder having a uniform depth. Then, using the spatula, the pharmacist cuts into the powder lengthwise and crosswise to delineate the appropriate number of smaller, uniform blocks, each representing a dose or unit of medication. Each of the smaller blocks is separated from the main block with the spatula, transferred to powder paper, and wrapped. The powder papers may be of any size convenient to hold the amount of powder required, but the most popular commercially available sizes are 2.75 × 3.75 inches, 3 × 4.5 inches, 3.75 × 5 inches, and 4.5 × 6 inches. The papers may be (a) simple bond paper; (b) vegetable parchment, a thin, semiopaque paper with limited moisture resistance; (c) glassine, a glazed, transparent paper, also with limited moisture resistance; and (d) waxed paper, a transparent waterproof paper.
The selection of the type of paper is based primarily on the nature of the powder. If the powder contains hygroscopic or deliquescent materials, the water-proof or waxed paper should be used. In practice, such powders are double wrapped in waxed paper, and then for esthetic appeal, they are wrapped in bond paper. Glassine and vegetable parchment papers may be used when only a limited barrier against moisture is necessary. Powders containing volatile components should be wrapped in waxed or glassine papers. Powders contain- ing neither volatile components nor ingredients adversely affected by air or moisture and are usually wrapped in a white bond paper.
Examples of Powder Formulations
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