Capsules

A capsule is a solid oral dosage form in which the drug and its excipients are enclosed within a soluble shell, most often made of gelatin or a plant-derived polymer such as hydroxypropyl methylcellulose. Capsules come in two principal types: hard-shell (two-piece) capsules that are filled with powders, granules, pellets, or even small tablets, and soft-shell capsules (softgels) that enclose liquids, suspensions, or semisolids.

Definition

A capsule is a unit oral dosage form consisting of a drug formulation enclosed within a soluble shell, presented either as a two-piece hard capsule filled with solids or as a one-piece soft capsule enclosing a liquid or semisolid fill.

Scope

This entry covers the construction of hard and soft capsules, the shell materials used, the kinds of fill they accommodate, and how capsules compare with tablets as a delivery format. It treats capsules as a formulation topic and is a reference overview rather than compounding or clinical guidance.

Core questions

• How do hard-shell and soft-shell capsules differ in construction and fill?
• What shell materials are used, and why are non-gelatin alternatives adopted?
• What kinds of fill — powders, pellets, liquids, semisolids — can a capsule hold?
• When is a capsule preferred over a tablet?

Key concepts

• Hard-shell (two-piece) capsule
• Soft-shell capsule (softgel)
• Gelatin and HPMC shells
• Powder, pellet, and liquid fill
• Multiparticulate fill
• Capsule size designations
• Liquid-filled and lipid-based fills

Mechanisms

Hard capsules are manufactured as empty two-piece shells that are filled with a metered quantity of powder, granules, pellets, or mini-tablets and then closed; soft capsules are formed, filled, and sealed in a single operation around a liquid or semisolid fill (Aulton & Taylor, 2018; Allen & Ansel, 2018). The shell dissolves or ruptures in gastrointestinal fluid to release the contents. Because the fill can be a liquid or a lipid-based vehicle, capsules are a common route for delivering poorly water-soluble drugs in solubilised or lipid systems, one of the enabling strategies framed by biopharmaceutics-based formulation design (Kawabata et al., 2011). Multiparticulate fills allow modified-release pellets to be combined within a single capsule.

Clinical relevance

Capsules are a familiar oral format whose shell and fill influence how a medicine is taken and stored, and shell composition can matter for patients with particular dietary or allergy considerations. This entry describes the dosage form and does not offer dosing or individualised administration advice.

Evidence & guidelines

Capsule products are tested against pharmacopoeial standards for dissolution and uniformity of dosage units, which provide the compendial basis for quality (USP, 2023); design principles are set out in standard pharmaceutics references (Aulton & Taylor, 2018; Allen & Ansel, 2018).

History

Gelatin capsules date to the early nineteenth century, when soft gelatin shells were devised to mask the taste of unpleasant medicines; hard two-piece capsules and industrial filling followed, and in recent decades plant-based shell polymers such as hydroxypropyl methylcellulose have been introduced as alternatives to animal gelatin (Aulton & Taylor, 2018; Allen & Ansel, 2018).

Seminal works

• aulton-2018
• allen-ansel-2018

Frequently asked questions

What is the difference between a hard and a soft capsule?

A hard capsule is a two-piece shell filled with solids such as powders or pellets and then closed, whereas a soft capsule is a single sealed shell that encloses a liquid or semisolid fill and is formed and filled in one operation.

Are all capsule shells made of gelatin?

No. Although gelatin is traditional, plant-derived polymers such as hydroxypropyl methylcellulose are widely used as non-animal alternatives for the capsule shell.