An excipient is an inactive substance used as a carrier for the active ingredients of a medication. In addition excipients can be used to aid the process by which a product is manufactured. In general, the active substances (such as aspirin) may not be easily administered and absorbed by the human body; they need to be put in some appropriate form. In such cases, the active substance is dissolved or mixed with an excipient. Excipients are also sometimes used to bulk up formulations with very potent active ingredients, to allow for convenient and accurate dosage.
Once the active ingredient has been purified, it cannot stay in purified form for very long. In many cases it will denature, fall out of solution, or stick to the sides of the container. To stabilize the active ingredient, excipients are added to ensure that the active ingredient stays active, and is stable for a long enough period of time that the shelf-life of the product makes it competitive with other products. The formulation of these excipients in many cases is considered a trade secret.
Pharmaceutical codes require that all ingredients in drugs, as well as their chemical decomposition products are identified and guaranteed to be safe. For this reason, excipients are only used when absolutely necessary and in the smallest amounts possible.
Types of excipients:
Binders hold the ingredients in a tablet together.
Binders ensure that tablets and granules can be formed with required mechanical strength. Binders are usually starches, sugars, cellulose or modified cellulose such as hydroxypropyl cellulose, lactose, or sugar alcohols like xylitol, sorbitol or maltitol.
Binders are classified according to their application:
- Solution binders are dissolved in a solvent (for example water or alcohol and used in wet granulation processes. Examples are Gelatin, Cellulose, Cellulose derivatives, Polyvinyl pyrrolidone, Starch, Sucrose and Polyethylene glycol
- Dry binders are added to the powder blend, either after a wet granulation step, or as part of a direct powder compression (DC) formula. Examples are Cellulose, Methyl cellulose, Polyvinyl pyrrolidone, Polyethylene glycol
Tablet coatings protect tablet ingredients from deterioration by moisture in the air and make large or unpleasant-tasting tablets easier to swallow. For most coated tablets, a cellulose (plant fiber) film coating is used which is free of sugar and potential allergens. Occasionally, other coating materials are used, for example synthetic polymers, shellac, corn protein zein or other polysaccharides.
Changing the dissolution rates of active species
Enteric coatings or slow release coatings control the rate of drug release, or determine where the drug will be released in the digestive tract.
- Water uptake facilitators
- Tablet rupture promoters
They ensure that when the tablet is in contact with water, it rapidly breaks down into smaller fragments, thereby facilitating dissolution. Examples of disintegrants include: starch, cellulose, crosslinked polyvinyl pyrrolidone, sodium starch glycolate, sodium carboxymethyl cellulosemethycellulose.
Fillers fill out the size of a tablet or capsule, making it practical to produce and convenient for the consumer to use. By increasing the bulk volume, the final product has the proper volume for patient handling.
A good filler must be inert, compatible with the other components of the formulation, non-hygroscopic, soluble, relatively cheap, compactible, and preferably tasteless or pleasant tasting.
Plant cellulose (pure plant filler) is a popular filler in tablets or hard gelatin capsules. Dibasic calcium phosphate is another popular tablet filler. A range of vegetable fats and oils can be used in soft gelatin capsules.
Flavors and Colors
Glidants are used to improve the flowability of the powder or granules or both.
Lubricants prevent ingredients from clumping together and from sticking to the tablet punches or capsule filling machine. Lubricants also ensure that tablet formation and injection can occur with low friction between the solid and die wall.
Some typical preservatives used in pharmaceutical formulations are
- antioxidants like vitamin A, vitamin E, vitamin C, retinyl palmitate, and selenium
- the amino acids cysteine and methionine
- citric acid and sodium citrate
- synthetic preservatives like methyl paraben and propyl paraben.
Sweeteners are added to make the ingredients more palatable, especially in chewable tablets such as antacid or liquids like cough syrup. Therefore, tooth decay is sometimes associated with cough syrup abuse. Sugar can be used to disguise unpleasant tastes or smells.