In a typical tablet the active ingredient accounts for a few percent of the mass. The rest is excipient. Excipients do not treat anything, but they decide whether the tablet can be compressed at all, whether it disintegrates in the right place, and whether the assay still holds after two years in the blister.
The definition
An excipient is any component of a formulation other than the active ingredient, included deliberately to perform a technical role: to provide bulk, bind granules, make the tablet disintegrate, reduce friction during compression, mask taste, exclude moisture, stabilise pH or control where the drug is released.
The key point: an excipient has no intended pharmacological effect. But "no pharmacological effect" does not mean "inert". Excipients interact with the active, affect bioavailability, and can cause reactions in some patients — lactose in intolerant patients being the familiar example.
Excipients are not vaccine adjuvants
This is a common confusion in Vietnamese-language material, including on some industry websites. Adjuvants — aluminium hydroxide, MF59, MPL, CpG — are added to vaccines to amplify the immune response. That is a deliberate biological effect, which is the opposite of what defines an excipient.
Vaccines do contain genuine excipients: stabilisers, buffers, preservatives, diluents. But when a text says "excipients increase the immune response" or "excipients reduce a drug's potency", it is describing adjuvants, and both statements are wrong if read as being about excipients.
Seven functional classes
The most useful way to classify excipients during formulation is by what they do, not by what they are chemically:
- Fillers and diluents
- Provide mass and volume when the active is present at low dose, while also shaping the powder's flow and compressibility.
- Binders
- Bind powder particles into robust granules. Too much and the tablet becomes hard and slow to disintegrate.
- Disintegrants
- Break the tablet apart in the gastrointestinal tract. Their effect depends on where they are added — intragranular, extragranular or split between both.
- Lubricants
- Reduce friction between tablet and die wall. They are hydrophobic, so over-blending slows disintegration and depresses dissolution.
- Glidants
- Improve powder flow so tablet weight stays uniform. Used at very low levels; above the optimum the effect reverses.
- Film coating systems
- Mask taste, exclude moisture, identify the product or control the site of release. Enteric coats must survive gastric pH yet dissolve promptly in the small intestine.
- Liquid dosage excipients
- Build a stable dispersion for syrups, suspensions and emulsions, together with preservation and pH adjustment.
Choosing by dosage form
- Tablets — typically need all six: filler, binder, disintegrant, lubricant, glidant and coating.
- Hard capsules — much simpler: filler, lubricant and glidant, since the shell already handles containment and taste masking.
- Powders and granules — filler, binder, glidant, plus sweetener and flavour.
- Liquids — suspending agent, emulsifier, preservative and pH adjuster.
Four questions before committing to an excipient
- Is it compatible with the active? This needs compatibility data, not inference from another formulation.
- Which pharmacopoeial monograph does it meet — USP, BP, EP, JP or IP? This goes into the registration dossier.
- Does the manufacturer supply full documentation? At minimum a batch CoA, MSDS and GMP certificate.
- Is supply reliable? Changing excipient supplier mid-lifecycle drags a revalidation along with it.