Tablets are solid dose pharmaceutical preparation containing drug substances usually prepared with the aid of suitable pharmaceutical excipients. They may vary in size, shape, weight, hardness, thickness, disintegration, and dissolution characteristics and in other aspects, depending on their intended use and method of manufacture.
It has been estimated that solid-dosage forms constitute approximately 90% of all dosage forms clinically used to provide systemic administration of therapeutic agents. The widespread use of tablets has been achieved as a result of their convenience and also the diversity of tablet types.
Tablets are prepared primarily by compression of granules or powder blends, with a limited number prepared by moulding. Most tablets are used in the oral administration of drugs. Many of these are prepared with colourants and coatings of various types. Other tablets, such as sublingual, buccal, or vaginal tablets, are prepared to have features most applicable to their particular route of administration.
General Properties of Tablets
- A tablet must be strong and hard to withstand mechanical shock during manufacturing, packing, shipping, dispensing and use.
- The drug content of the tablet must be bioavailable that is, the tablet must be able to release its content in a predictable and reproducible manner.
- The tablet must be chemically and physically stable to maintain its chemical and physical attributes during manufacture, storage, and use.
- The tablet should have elegant product identity which is free from any tablet defect.
- Tablets must be uniform in weight and in drug content.
Types of tablets
The various tablet types are described as follows:
a. Compressed tablets
Compressed tablets represent a significant proportion of tablets that are clinically used to provide systemic administration of therapeutic agents either in an uncoated state (i.e., in their simplest form) or in a coated state. These tablets are designed to provide rapid disintegration in the gastric fluid following ingestion hence, allowing rapid release of the drug and, ultimately, systemic absorption of the dosage form.
Compressed tablets are formed by compression of powdered, crystalline, or granular materials into the required geometry by the application of high pressures, utilizing steel punches and die. In addition to the Active Pharmaceutical Ingredient(s) (APIs), compressed tablets usually contain a number of pharmaceutical excipients e.g., bulking agents, disintegrants, binders, lubricants, controlled-release polymers and other miscellaneous adjuncts such as colourants and flavourants which serve different and specialized purpose during tablet manufacture, storage, and use. Examples of compressed tablets include tablets for oral, buccal, sublingual, or vaginal administration.
b. Sugar-coated Tablets
These are compressed tablets that have been coated with concentrated sugar solution to improve patient’s compliance, increase aesthetic appeal, mask objectionable tastes or odours, increase stability and/or modify the release of therapeutic agent(s). Sugarcoating was once quite common but lost commercial appeal due to the time and expertise required in the coating process, the increase in size and weight of coated tablets, high cost of process validation and shipping.
The advent of film-coated tablets has also greatly decreased the use of sugar coatings due to the improved mechanical properties of the technique. Examples of sugar-coated tablets include Reasulf tablets – dried ferrous sulphate BP 200mg (Reagan Remedies Ltd.), Advil – Ibuprofen tablet BP 200mg (Pfizer Consumer Healthcare), Ebu-200 – Ibuprofen tablet BP 200mg (Me cure Industries Ltd) etc.
c. Film-Coated Tablets
Film-coated tablets are conventional tablets coated with a thin layer of polymer (e.g., hydroxypropyl methylcellulose, hydroxypropyl cellulose) or a mixture of polymers (e.g., Eudragit E100) capable of forming a skin-like film. The film is usually coloured and also impacts the same general characteristics as sugar coating with the added advantage of being more durable, less bulky, and less time-consuming to apply. By its composition, the coating is designed to break and expose the core tablet at the desired location in the gastrointestinal tract.
Advances in material science and polymer chemistry have made these coatings the first choice for formulation scientists. Examples of Film-coated tablets include Curefenac 100 – Diclofenac potassium USP 100mg (Unicure Pharmaceutical Ltd), Valsartan 320mg Film-coated Tablets (Actavis UK Ltd), etc.
d. Effervescent Tablets
Effervescent tablets are uncoated tablets that generally contain organic acids (such as tartaric or citric acid) and sodium bicarbonate in addition to the medicinal substance or API. They react rapidly in the presence of water by releasing carbon dioxide which acts as a disintegrator to produce either a drug suspension or an aqueous solution. These tablets are prepared by compressing granular effervescent salts (organic acid and bicarbonate) with the medicinal substances. A typical example of this tablet type is Ca C1000 Sandoz effervescent tablet (Novartis).
e. Enteric-coated Tablets
Enteric-coated tablets are compressed tablets that have delayed-release properties. They are coated with polymeric substances (such as cellulose acetate phthalate/cellulose acetate butyrate; hydroxypropylmethylcellulose succinate; and methacrylic acid copolymers) that resist solution in gastric fluid but disintegrate and allow drug dissolution and absorption in the intestine.
Enteric coatings are primarily employed when the drug substance is inactivated or destroyed by gastric acid (e.g., erythromycin) or is particularly irritating to the gastric mucosa (e.g., non-steroidal anti-inflammatory drugs) or when bypass of the stomach substantially enhances drug absorption. Example of enteric-coated tablets includes Lofnac 100 – Diclofenac sodium delayed-release tablet USP 100mg (bliss GVS Pharma Ltd), Ecotrin tablets and caplets (GlaxoSmithKline Beecham).
f. Chewable Tablets
Chewable tablets are big sized tablets which are difficult to swallow and thus, are chewed within the buccal cavity prior to swallowing. They are especially useful for administration of large tablets to children and adults who have difficulty swallowing conventional tablets or antacid formulations in which the size of the tablet is normally large and the neutralisation efficacy of the tablet is related to particle size within the stomach.
Chewable tablets are not conventionally used if the drug has issues regarding taste acceptability. Examples of chewable tablets include Danacid – compound magnesium trisilicate tablet B.P. (Dana Pharmaceuticals Limited), Gestid – tasty chewable antacid (Ranbaxy) etc.
g. Buccal and Sublingual Tablets
Buccal and sublingual tablets are small, flat, oval tablets that are intended to be dissolved in the buccal pouch (buccal tablets) or beneath the tongue (sublingual tablets) for absorption through the oral mucosa to produce a systemic effect. These tablets are employed to achieve either rapid absorption into the systemic circulation e.g. glyceryl trinitrate sublingual tablets or, alternatively, to enable oral absorption of drugs that are destroyed by the gastric juice and/or are poorly absorbed from the gastrointestinal tract.
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