Definition and Characteristics of Capsules
1. Definition of Capsules
Capsules are solid preparations made by filling pharmaceutical ingredients or excipients into empty capsules or by sealing them in soft capsule materials. They are mainly for oral use.
2. Characteristics of Capsules
Capsules are one of the primary dosage forms for oral administration. Currently, among pharmaceutical dosage forms, capsules rank third, after tablets and injections. As an oral medication, capsules have the following characteristics:
(1) Fast drug dissolution rate and high bioavailability. Compared to tablets and pills, capsules do not require the addition of binders or compression during preparation, allowing them to disperse quickly and be well absorbed in gastrointestinal fluids.
(2) Masking unpleasant odors and high drug stability. Because the drug is encapsulated in a capsule, it has a masking effect on drugs with bitter or irritating odors; for medicines that are photosensitive or unstable in humid and hot conditions, it can prevent them from being affected by moisture, oxygen, and light in the air, thus playing a protective and stabilizing role.
(3) Capsules can compensate for the shortcomings of other solid dosage forms: Drugs with high oil content or liquid form that are difficult to make into tablets can be made into capsules; drugs with small dosages, poor water solubility, and poor gastrointestinal absorption can be dissolved in suitable oils and then made into capsules, which not only increases gastrointestinal absorption and improves efficacy but also enhances stability.
(4) Capsules can delay or target drug release: Drugs can be made into granules, then coated (Micro pellets), and filled into empty capsules to create various types of capsules, such as sustained-release, controlled-release, and enteric-coated capsules.
(5) Capsule shells are neat in appearance, easy to identify, and easy to carry. Capsule shells come in various colors or with printed text, which facilitates identification and makes them aesthetically pleasing, convenient for administration, and easy to carry.
3. Drugs Unsuitable for Capsule Formation
Because the main component of capsules is gelatin, which is brittle and water-soluble, the following conditions are unsuitable for capsule formation:
(1) Aqueous solutions or dilute ethanol solutions of drugs: Water and ethanol can dissolve the capsule wall and are not suitable for filling empty capsules.
(2) Hygroscopic Drugs: Highly hygroscopic drugs can cause capsule walls to dry out and crack. However, this can be mitigated by adding a small amount of inert oil and mixing it with the hygroscopic drug, thus delaying or preventing the capsule wall from becoming brittle and producing a capsule formulation.
(3) Efflorescent Drugs: The water of crystallization released after the drug effloresces can soften the capsule wall, affecting the quality of the capsule formulation.
(4) Easily Soluble Drugs: Drugs such as chlorides, bromides, iodides, and small doses of irritating drugs can dissolve in the stomach, leading to excessively high local concentrations and irritating the gastric mucosa.
(5) Strong Acids and Alkalis: Acidic liquids can hydrolyze gelatin, causing leakage; alkaline liquids can tanninate gelatin, affecting its solubility.
3. Classification of Capsules: Capsules can be classified into the following categories based on their hardness, solubility, and release characteristics.
(1) Hard Capsules
Hard capsules (commonly referred to as capsules) are capsules made by filling empty capsules with a raw material, drug, or a suitable excipient, prepared into a uniform powder, granules, tablets, microspheres, semi-solids, or liquid, using appropriate formulation technology.
(2) Soft Capsules
Soft capsules are capsules made by directly sealing a certain amount of liquid raw material drug, or by dissolving or dispersing a solid raw material drug in a suitable excipient to prepare a solution, suspension, emulsion, or semi-solid, and then sealing it in a soft capsule material.
(3) Enteric-Coated Capsules
Enteric-coated capsules are hard capsules made by filling capsules with granules or microspheres coated with enteric-coating materials, or hard or soft capsules prepared with suitable enteric-coating materials. Enteric-coated capsules are insoluble in gastric juice but disintegrate in intestinal juice, releasing the active ingredient.
(4) Sustained-Release Capsules
Sustained-release capsules release the drug slowly and at a non-constant rate in a specified release medium. (5) Controlled-Release Capsules
Controlled-release capsules refer to capsules that release drugs slowly and at a constant rate in a specified release medium.
4. Packaging and Storage
Capsules are typically packaged using materials such as glass bottles, plastic bottles, blister packaging, and strip packing.
The gelatin raw material in capsules is unstable under high-temperature, high-humidity conditions. The Shell absorbs moisture, softens, becomes sticky, swells, and may even melt or dissolve, potentially leading to microbial growth. When the environment is too dry, the capsules easily lose moisture and become brittle. Therefore, unless otherwise specified, capsules should be stored in sealed containers at a temperature not exceeding 30°C and with suitable humidity to prevent moisture absorption and mold growth.
If blister packaging is used, alu-alu, alu-PVC-alu, or a combination of blister packaging and pillow packaging can significantly enhance the barrier effect and extend the drug’s shelf life.