Tablets are known as single unit solid dosage forms, containing fixed quantity of drug. As per United State Pharmacopoeia (USP), tablets are defined as “Solid dosage forms, containing medicinal substances with or without suitable diluents and they may be prepared by compression or moulding”. According to Indian Pharmacopoeia (IP), “Pharmaceutical tablets are solid, flat or biconvex dishes, unit dosage forms, prepared by compressing a drug or a mixture of drugs, with or without diluents”.
The preparation of tablets for large scale production is exclusively achieved by compression method, while small scale production can be carried out by both the methods compression and moulding.
A well-defined compressed tablet possesses following attributes:
In addition to the Active Pharmaceutical Ingredient (API Drug), tablets may contain a number of other inert materials, these additional materials are known as “formulation additives or excipients”.
These formulation excipients play different roles in tablet formulation and to attain its various desirable characteristics i.e., content uniformity, tablet formulation, tablet dissolution, disintegration, drug release etc.
The main categories of excipients are:
The actual therapeutic dose of a drug is very small and which can’t be compressed alone to form tablets. Thus some extra inert substances which does not affect drug’s action and therapeutic effectiveness are added with drugs to produce bulk.
This combination of drug (in small quantity) and diluent (in higher quantity) produces bulk, which can be easily compressible.
The diluent should be:
• Physiologically inert, physically & chemically stable by themselves & in combination with the drugs.
• They must be free from all microbial contamination.
• They do not alter the bioavailability of drug.
Examples: > Lactose, Cellulose, Kaolin, Mannitol, Dry starch, Dibasic calcium phosphate, Calcium sulfate, Sodium chloride, Sorbitol, Microcrystalline cellulose.
The substances that “glue” or “adhere” the powder particles together and cause them to form granules, are called binders or adhesives. They provide cohesiveness to the powdered particles and granules, to ensure the intactness of tablet after compression.
Examples:
Lubricants have number of functions in tablet manufacturing. They prevent adhesion of the tablet material to the surface of dies and punches to, also facilitates ejection of tablets from die cavity, reduce inter particle frictions and improves the flow of powder granules.
In fact, the term “lubricant” is used to describe three different functions.
1. The lubricant can promote particulate flow, so that a reproducible die fill is obtained and hence there is a uniformity of tablet weight. The term “glidant” is used to describe this function.
2. The lubricant can prevent the punch faces from sticking to the faces of the tablet as the latter is ejected from the die. This is better described as an “anti-adherent” action.
3. The lubricant can prevent adhesion between the sides of the tablet and the die wall as the tablet is pushed out of the die by the ascending lower punch.
Lubricants usually are added at the very last step of tablet granules, before compression, so that they must be present on the surface of granules and on the parts of tablet press.
Anti-adherents are the subtype of lubricants, which can reduce sticking the tablet to the punch face. The most common example of this is Talc, the concentration depends on the formulation, but the general range used between 1-5%.
Examples: Talc, magnesium stearate, Calcium stearate, Stearic acid, Hydrogenated vegetable oils and Poly ethylene Glycol (PEG) etc.
| Material as Lubricant | Range Commonly used in % |
|---|---|
| Magnesium Stearate | 0.25 – 5 |
| Calcium Stearate | 0.5 – 1 |
| Sodium lauryl sulfate | 1 – 2 |
| Zinc Stearate | 0.5 – 2 |
| Polyethylene Glycol | 2 – 15 |
| Sodium Stearyl Fumarate | 0.5 – 2 |
| Stearic acid | 0.1 – 2 |
| Sodium benzoate | 5 |
They assist the disintegration of the tablet and can be act by one of the two main mechanisms.
• One is to act as a water-soluble path for the water to penetrate into compact dosage form.
• The second one is by swelling up and applying pressure, which breaks apart the tablet.
A tablet disintegrant may be defined as: Any/ solid, pharmaceutically acceptable material included in the formulation that acts to cause the tablet matrix to break up when the tablet comes into contact with aqueous media.
Disintegrating agents added to tablets for the causing them to break apart (disintegrate), when placed in aqueous media. Basically, these agents are work to oppose the action of binders.
| Material | Range used in % |
|---|---|
| Cellulose | 1 – 10 |
| Cross Linked PVP | 2 – 5 |
| Croscarmellose Sodium | 0.5 – 5 |
| Sodium Starch Glycolate | 2 – 8 |
| Starch | 2 – 10 |
Some agents work as binders and disintegrating agents, but the basic difference is in the concentration, as:
Starch when used in the concentration range of 1-4%, it works as binder, but when it is used in the concentration range of 2-8%, it works as disintegrating agent. Gums as binders in the range of 1-3%, but works as disintegrating agent in the range of 1-10% of tablet weight.
Examples- Sodium carboxy methyl starch, natural and synthetic gums (locust bean gum, karaya gum, guar gum, tragacanth gum etc.)
Cellulose derivatives- microcrystalline cellulose (over 25% of tablet weight).
Alginates (sodium alginate at 5-10%), Bentonite clay and Veegum.
An effervescent base can be used to promote disintegration by the liberation of gas. This formulation has citric or tartaric acid together with sodium bicarbonate, potassium bicarbonate or calcium carbonate. These react in contact with water to liberate carbon dioxide which disrupt the tablet.
Many tablet formulations are cohesive, and their constituents can stick together for a variety of reasons. A fundamental cause of cohesion is the presence of attractive forces between adjacent particles. These cohesive forces may prevent uniform flow of the formulation, and it is the function of the glidant to improve flow so that specifications on uniformity of tablet weight can be met.
Glidants reduces friction between particles so the powder flows smoothly into tablet dies or capsule shells.
| Material as Glidant | Concentration in Tablet in % |
|---|---|
| Calcium silicate | 0.5 – 2 |
| Powdered Cellulose | 1 – 2 |
| Colloidal Silicon | 0.05 – 0.5 |
| Magnesium Oxide | 1 – 3 |
| Magnesium Silicate | 0.5 – 2 |
| Starch | 2 – 10 |
| Talc | 1 – 10 |
Colouring agents are added to provide aesthetic values (to provide appealing look) to the tablet. Colouring to the tablets is necessary for elegancy and mainly to differentiate tablets containing different drugs or to differentiate the tablets of different brands.
Colouring agents must be approved from various Drug and Food Administrations (FDA). The most common way to add colouring agents is to dissolve it into solution of binder and then blended it with powder for granulation. The maximum concentration of dyes is 0.03%.