The range of tablet diluent may vary from 5-80%. Diluents are also synonymously known as fillers. Diluents are often added to tablet formulations to provide better tablet properties such as:
  1. To improve cohesion
  2. To allow direct compression manufacturing
  3. To enhance flow
  4. To adjust weight of tablet as per die capacity

Diluent/filler for tablet must meet some criteria. They are as follows:
  • Diluent should not react with the drug substance and moreover it should not have any effect on the functions of other excipients
  • it should not have any physiological or pharmacological activity of its own
  • it should have consistent physical and chemical characteristics
  • it should neither promote nor contribute to segregation of the granulation or powder blend to which they are added
  • it should be able to be milled (size reduced) if necessary in order to match the particle size distribution of the active pharmaceutical ingredient
  • it should neither support microbiological growth in the dosage form nor contribute to any microbiological load
  • it should neither adversely affect the dissolution of the product nor interfere with the bioavailability of active pharmaceutical ingredient
  • it should preferably be colourless or nearly so.
Classification of Tablet Diluents/Filler 

Tablet diluents or fillers can be divided into three categories:

i)                    Organic materials - Carbohydrate and modified carbohydrates:
·         Sucrose, Manitol, Sorbitol
ii)                  Inorganic materials
·         Calcium phosphates : Anhydrous Dibasic Calcium Phosphate, Dibasic Calcium Phosphate, Tribasic Calcium Phosphate
iii)                Co-processed Diluents

Carbohydrate substances such as sugars, starches and celluloses may also function as binders during wet granulation process. Whereas when used in direct compression system, they serve as the diluent. The inorganic diluents, do not exhibit binding properties when used in wet granulation and direct compression.
Tablet diluent or filler may also be classified on the basis of their solubility in water as soluble and insoluble.

Classification of Diluents Based On Their Solubility

Insoluble Tablet Filler or DiluentsSoluble Tablet Filler or Diluent
Powdered CelluloseSucrose
Microcrystalline CelluloseManitol
Calcium PhosphateSorbitol

Selection of diluent should be done after considering properties of diluent such as:
  • Compactibility
  • Flowability
  • Solubility
  • disintegration qualities
  • hygroscopicity
  • lubricity
  • stability