A dosage form is the physical form in which a therapeutic agent is manufactured, dosed, or administered. Examples of dosage forms include capsules, tablets, powders, suspensions, injectables, etc.
Capsules have become a dominant dosage form option in the pharmaceutical and nutraceutical industry for oral medication delivery. This is due to their better versatility in holding varieties of active ingredients (solids, liquids, or semisolids) for oral use compared to tablets. Capsules also possess a stronger capacity to mask the offensive tastes of drugs and to support attractive branding of products. These advantages have secured a favorite spot for capsules with manufacturers and consumers alike.
There are, nonetheless, subdomains of dosage forms under capsules as an umbrella term, which speaks to the specialized applications and functions capsules can serve. Understanding these options as a health-conscious consumer or manufacturer can guide your choice and maximize the benefits of capsules for your health or brand.
Keep reading to learn about the 4 common variants of the capsule dosage form, their peculiarities, unique applications, and benefits.
1. Hard capsules
These are a two-piece capsule system consisting of separable cylindrical halves – the body and the cap– for easy filling and locking purposes. Gelatin hard caps are the most common type of hard capsules on the market. They are made from pharmaceutical-grade bovine, porcine, or piscine gelatin via the 'dip-coating' technique. However, hydroxypropyl methylcellulose (HPMC) and pullulan capsules are also top-rated vegetarian alternatives to gelatin capsules in the hard capsule section.
Hard caps are usually prefabricated and supplied as unlocked empty capsules to pharmaceutical manufacturers and home users to be filled with drugs, herbs, or nutraceuticals for use.
Hard empty capsules are amenable for use with a wide array of ingredients. These include dry solids (powders, granules, pellets, tablets, etc.), semi-solids (pastes and suspensions), and liquids (aqueous and non-aqueous solutions and oils)
However, dry solids and oils may require the incorporation of other encapsulation excipients, e.g. diluents, disintegrants, wetting agents, etc. to maintain the stability of the fill materials and release profile of the capsules. Also, users and manufacturers should match the properties of the filler ingredients to their choice of hard capsules.
For example, while both gelatin and HPMC vegetarian capsules are highly durable and versatile, hard gel caps are more sensitive to hygroscopic materials than HPMC capsules, and may also contend with religious and dietary restrictions among a section of the consumers. HPMC capsules, on the other hand, may scale these hurdles but are more expensive and may result in higher production costs.
Soft gelatin capsules or softgels are a one-piece hermetically-sealed system with a solid outer shell and semi-solid or liquid core – one or both of which may hold the active ingredients. They are produced with the fill incorporated at the point of manufacture in a rotary capsule machine or dosator. A softgel is composed of gelatin, 6-13% by weight of water, 20-30% of plasticizers e.g. sorbitol and glycerin, opacifiers, and colorants. Plasticizers aid the stability of the fill and slow the migration of materials between the shell and core. Optional excipients employed include preservatives, solubilizers, suspending agents, and wetting agents.
Softgels are made in a variety of shapes and sizes and serve different functions per time. They also have good aesthetic appeal and high consumer acceptance. While some are designed to be swallowed whole for the release of content, fill materials in twist-off and suckable softgels can be consumed directly as found with some vitamins.
Generally, softgels are thicker than hard capsules but they also dissolve faster and release their content more rapidly. Although, such rapid release rates might result in side effects associated with a 'dump' or high concentration of active ingredients like abdominal disturbances, nausea, and vomiting.
Fill materials that can be incorporated in a soft gelatin capsule include pure oils, oily solutions or mixtures, and micronized suspensions and pastes. Aqueous solutions, extremely water-soluble drugs, and moisture-sensitive materials are not suitable for softgels due to their high moisture content.
3. Modified-release capsules
Modified-release dosage forms are employed to control the pattern, location, or time of release of the active pharmaceutical ingredients from a formulation. They can also be used to maintain the level of active drugs in the body over a period, via a time-dependent release. Capsules are inherently immediate-release dosage forms but can be modified into a controlled-release system, with enhanced or delayed release rates.
The rate of drug release of hard gelatin capsules can be enhanced by using hydrophilic excipients in the fill or installing micro-vents containing citric acid and sodium bicarbonate in the outer film of hard gels. A delayed-release variant of the hard capsules can be achieved by coating the shell with polymers such as glucomannan and polyvinyl pyrrolidone. HPMC phthalate esters (HPMCP) can also be used to modify the release rates of both hard gelatin capsules and HPMC vegetarian varieties. Microencapsulation techniques with pellets and mini-tablets can also help modify the release rates of hard caps.
To enhance the disintegration and solubility of soft capsules, a small concentration of surfactants like sodium lauryl sulfate may be incorporated into the capsule. Where a delayed-release rate is desired, heavy polymers and alginates are added to the capsule matrix instead.
Delayed-release capsules dissolve slowly over many hours. Hence, they may help minimize the local gastric irritation or other systemic side effects of medications due to sudden peak blood levels that are associated with immediate-release caps. Other benefits of delayed-release capsules include improved half-life due to sustained release, lesser frequency and convenience of dosing, and improved user compliance.
4. Acid-resistant capsules
These are more specialized forms of modified-release capsules. Acid-resistant capsules (also called enteric capsules) are designed to selectively deposit their content in the small intestine after passing through the stomach unbroken. They are used to hold fill materials e.g. drugs, supplements, probiotics, etc that are prone to degradation by stomach acid or are likely irritative to the gastric mucosa.
Acid resistance is built in the capsules by coating a finished capsule or by direct incorporation of the enteric polymer during the production process. Common enteric coatings that have been employed include mixtures of waxes, fats, and esters of fatty acids, cellulose acetate phthalate, HPMCP, etc. Most of these polymers work in a pH-dependent fashion i.e. they are insoluble in acid but soluble in alkali. Some neutral polymers also erode over time in the gut without the pH effect.
Enteric capsules are also amenable to a wider variety of fill ingredients including oils, semi-solids, and inert hydrophobic solvents. However, given the sophistication and higher cost associated with the enteric coating technique, they are often reserved only for acid-labile materials or those with similar critical nature.
The various capsule dosage forms demonstrate the remarkable flexibility of capsules to optimize medication delivery. This is coupled with the opportunity to customize the sizes, shapes, flavors, and colors. As such, pharmaceutical and supplement manufacturers as well as home users can find a suitable match for their products or meet specific needs.