Pharmaceutical excipient means any component other than the pharmacologically active drug which are included in the manufacturing process or are contained in a finished pharmaceutical product dosage form.

While selecting excipients for any formulation following things should be considered wherever possible: keep the excipients to a minimum in number minimize the quantity of each excipients and multifunctional excipients may be given preference over unifunctional excipients.

Fewer ingredients in the formulation are better for the following reasons:
  • Excipients are not completely inert. Even commonly used excipients that are deemed to be pharmaceutically inactive and nontoxic may cause adverse reactions;
  • Less ingredient variability to influence process and product consistency;
  • Better economic efficiency in product manufacturing;
  • Less probability of chemical or physical interaction between API and excipients and among excipients.
Excipients play a crucial role in design of the delivery system, determining its quality and performance. Excipients though usually regarded as nontoxic there are examples of known excipient induced toxicities which include renal failure and death from diethylene glycol, osmotic diarrhoea caused by ingested mannitol, hypersensitivity reactions from lanolin and cardiotoxicity induced by propylene glycol.

Excipients may also be important for keeping the drug from being released too early in the assimilation process in places where it could damage tender tissue and create gastric discomfort or stomach upset.

Excipient Grades. Many excipients for pharmaceutical use are available in different grades. These grades are differentiated frequently by means of physical and chemical characteristics. The reason for grades is to change the performance characteristics of excipients.

Excipients are chosen in tablet formulation to perform a variety of functions like
  1. For providing essential manufacturing technology functions (binders, glidants, lubricants may be added),
  2. For enhancing patient acceptance (flavors, colourants may be added),
  3. For providing aid in product identification (colourants may be added),
  4. For Optimizing or modifying drug release (disintegrants, hydrophilic polymers, wetting agents, biodegradable polymers may be added),
  5. For enhancing stability (antioxidant, UV absorbers may be added)
Excipient With Their Functions In Tablet Formulation

Diluents or Fillers Diluents make the required bulk of the tablet when the drug dosage itself is inadequate to produce tablets of adequate weight and size.
Binders or Granulating
agents or Adhesives
Binders add cohesiveness to powders, thus providing the necessary bonding to form granules, which under compaction form a cohesive mass or a compact which is referred to as a tablet.
Disintegrants Disintegrant is added to facilitate a breakup or disintegration of the tablet when placed in an aqueous environment.

Antifrictional Agents
Lubricants Lubricants are intended to reduce the friction during tablet formation in a die and also during ejection from die cavity.
Antiadherents Antiadherents reduce sticking or adhesion of any of the tablet granulation or powder to the faces of the punches or to the die wall.
Glidants Glidants promote the flow of tablet granulation or powder mixture from hopper to the die cavity by reducing friction between the particles.

Wetting agents Wetting agents are added to tablet formulation to aid water uptake during
disintegration and assist drug dissolution.
Dissolution retardants Dissolution retardants as the name suggest, retards the dissolution of active
pharmaceutical ingredient(s).
Dissolution enhancers Dissolution enhancers as the name suggest, enhance the dissolution rate of active pharmaceutical ingredient(s).
Adsorbents Adsorbents are capable of retaining large quantities of liquids without becoming wet; this property of absorbent allows many oils, fluid extracts and eutectic melts to be incorporated into tablets.
Buffers Buffers are added to provide suitable micro environmental pH to get improved stability and / or bioavailability.
Antioxidants Antioxidants are added to maintain product stability, they act by being preferentially oxidized and gradually consumed over shelf life of the product.
Chelating agents Chelating agents are added to protect against autoxidation; they act by forming complexes with the heavy metal ions which are often required to initiate oxidative reactions.
Preservatives Preservatives are added to tablet formulation in order to prevent the growth of micro-organisms.
Colours Colours are added to tablet formulation for following purposes: to disguise off colour drugs, product identification and for production of more elegant
Flavours Flavours are added to tablet formulation in order to make them palatable enough in case of chewable tablet by improving the taste.
Sweeteners Sweeteners are added to tablet formulation to improve the taste of chewable tablets.

Drug-Excipient and Excipient-Excipient Interaction 

Interaction between drugs and excipients can occur by means of several possible mechanisms, including adsorption, complexation, chemical interaction, pH effects and eutectic formation resulting in drug products with desired or undesired properties.

E.g. Alkalinizing agents (Sodium Bicarbonate, Sodium Carbonate, Magnesium Oxide, etc.) and Acidifiers (Citric Acid, Tartaric Acid, Malic Acid, Fumaric Acid, etc.) can influence the microenvironment pH significantly and may have major influence on drug solubility for acidic and basic drugs. An excipient-excipient interaction sometimes can be used as a formulation strategy to achieve desired product attributes. (E.g. the viscosity of Xanthan gum is increased in the presence of Ceratonia)

Formulators must consider the following factors while designing holistic formulations– physiochemical properties, stability and compatibility issues, pharmacokinetic attributes, permeation characteristics, segmented absorption behavior, drug delivery platforms, intellectual property issues and marketing drive.