Tablets are classified on the basis of shape and size, process involved in its preparation and route and other characteristics.
Route and other Special Characteristics
Compounding of tablets requires following additives, in addition to the drug substance.
Diluents are substances to increase bulk and convert in the compressible form, when drug material is potent or inadequate to provide a suitable shape and size to tablet.
A tablet diluent must be compatible, inert, economic, easily available and organoleptically acceptable, should not affect the bioavailability of a drug adversely.
Examples of tablet diluents include dibasic calcium phosphate, calcium sulphate, lactose, lactose anhydrous, lactose spray dried, mannitol, sorbitol, sucrose, dextrose etc.
Examples of directly compressible diluents include Sta-Rx-1500, Emdex (contains dextrose 90 to 92% a maltose 3 to 5%), Celutab (dextrose & maltose), Avicel (Microcrystalline cellulose), Di-C (Dicalcium phosphate dihydrate), Cab-O-Sil (Colloidal silica).
Binders and Adhesive:
These materials are used in dry or liquid form to reduce the amorphous nature of substance and convert into compressible form (wet granulation).
Example include acacia, tragacanth, gelatin, alginates, methylcellulose, hydroxypropyl- methylcellulose, hydroxypropylcellulose, PVP, Starch, sorbiol, ethylcellulose, pregelatinized starch, glucose, iris moss, ghatti gum, arabogalactan, waxes, etc.
Most of the tablets contain disintegrating agent. Disintegrating agents facilitate the disintegration of the tablet in small particles in the gastrointestinal tract. Breaking of tablet' based on the swellability, adsorption of water or chemical reaction.
Examples include soluble starch, pre-gelatinized starch (PGS), veegum HV, bentonite, microcrystalline cellulose, sodium carboxy methylcellulose, PVP, guar gum, Isapgul, primogel, explotab, aerosil, natural spon citrus pulp, Alginic acid and alginates, Ion exchange resin, magnesium aluminium silicat modified corn starch, sodium dodecyl sulphate, sodium starch glycollate, etc.
Glidants act as a flow promoter and reduce the friction between particles. It improves the flow properties of granules or powder through hopper to die. It is not deformed compression pressure of the tablet machine. Examples include talc, starch, magnesium stearate, calcium stearate, boric acid, sugar, lycopodium and sodium chloride.
Lubricants reduce inter-particular friction. It improves the ejection of tablet from die wall and reduces the sticking problems and smooth tablets are produced. Examples include Talc, magnesium stearate, calcium stearate, stearic acid, polyethyleneglycols, starch derivative
Antiadhesives or Antisticking agents:
These materials are used to reduce the adhesion of the tablet surface to dies and punches during the compression of tablets. The pressure of the machine deforms these materials. It reduces sticking, picking and chipping problems. Examples include paraffin, stearic acid, cocoa butter, soaps, starch derivatives.
Colours are selected from 'permitted' list and are added to promote elegance and also to mask differences in colour or speckling when either the drug or an additive is off white.
Pastel shades are commonly used as these shades help in achieving uniform colour distribution. Coloring materials or dyes are used in tablet formulation mainly for three purposes; disguising of off color drugs, product identification and production of more elegant products. Colours may be added either to the vehicle used for granulation or to the mixture of powders prior to granulation.
The first approach is known to give better results provided migration of dye to the top of granules along with solvent during drying does not occur. When wet granulation is not to be employed, lake dyes (dyes adsorbed on alumina or aluminium hydroxide) are recommended.
Fading of the colour on standing and exposure to light leading to mottling of tablets is the common problem with dyes. Examples include water soluble dyes and many other FD&C approved colors or dyes.
These substances are not necessary for the formulation of compressed tablets. The proportion of flavours should generally be limited to 0.5% because excessive amounts may interfere with the free flow or cohesion of the granules.
Special tablets require flavoring agents such as chewable tablet, lozenges, etc. Examples include flavoring oils like cinnamon, coriander, and caraway etc.
Chewable tablets have sweetening agents because such tablets remain in the mouth and are not swallowed. Examples include saccharin sodium, aspartame, sugar, etc.