Capsules are the dosage forms in which the drug formulation in a powder, semisolid, or liquid form is enclosed in a shell. This shell is generally made from gelatin but can be made from other polymers such as hydroxypropyl methylcellulose (HPMC), polyvinyl alcohol (PVA), seaweed, or starch.
Capsules are usually colorless. A variety of colored capsules are now available in commerce. Colored capsules may be used to encapsulate ingredients that would appear unattractive in a colorless capsule.
Depending on the composition of the gelatin shell, the capsules can be hard or soft gelatin capsules. Soft gelatin capsules are made from a relatively more flexible, plasticized gelatin film than hard gelatin capsules.
Hard capsules, such as hard gelatin or HPMC capsules, are typically used for powder or solid fills, whereas soft gelatin capsules are used for semisolid or liquid fills. Lately, hard capsules have also been used for liquid or semisolid fills.