Types of tablets

Types of tablets

Depending on the physicochemical properties of the drug, site and extent of drug absorption in the gastrointestinal (GI) tract, stability to heat, light, or moisture, biocompatibility with other ingredients, solubility, and dose, the following types of tablets are commonly formulated (Table 20.1):

1. Swallowable tablets

The most common types of tablets are swallowed whole. These tablets dis-integrate and release their contents in the GI tract.

2. Effervescent tablets

These tablets are formulated to allow dissolution or dispersion in water prior to administration and should not be swallowed whole. In addition to the DS, these tablets contain sodium carbonate or bicarbonate and an organic acid such as tartaric acid. In the presence of water, these additives react, liberat-ing carbon dioxide, which acts as a disintegrator and produces effervescence. The drug is released into the aqueous medium as a solution, if it is highly soluble, or suspension. Ingestion of a dissolved or finely dispersed drug pro-vides a rapid rate of drug absorption. Therefore, effervescent tablets can be suitable for acute conditions that require immediate relief, such as pain and gastric acidity. For example, cephalon’s fentanyl effervescent tablet can be used to reduce the intensity of breakthrough pain in cancer patients.

3. Chewable tablets

Chewable tablets are used when a faster rate of dissolution and/or buc-cal absorption is desired. Chewable tablets consist of the drug dispersed throughout a saccharide base that provides mild sweetness. Flavors, sweet-eners, and colors are also added to chewable tablets to improve palatability and organoleptic appeal. The drug is released from the dosage form by physical disruption associated with chewing and dissolution in the fluids of the oral cavity, and the presence of a effervescent material. For example, some antacid tablets can be chewed to obtain quick indigestion relief. Chewable tablets are typically prepared by compression and usually contain mannitol or sorbitol as saccharide, mildly sweet, fillers. Mannitol is some-times preferred as a chewable base diluent, because it has a pleasant cooling sensation in the mouth due to negative heat of dissolution and can mask the taste of some objectionable medicaments.

Table 20.1 Types of tablets

4. Buccal and sublingual tablets

Buccal and sublingual tablets dissolve in the cheek pouch (buccal) or under the tongue (sublingual). Buccal or sublingual route of drug absorption bypasses hepatic metabolism, often referred to as the first-pass effect on oral administra-tion, and is preferred for low dose drugs that have extensive hepatic metabo-lism. Sublingual administration also allows rapid drug absorption, which may be critical in cases such as nitroglycerin for chronic heart failure. Other exam-ples include isoprenaline sulfate (bronchodilator), glyceryl trinitrate (vasodila-tor), and testosterone tablets. These tablets are usually small and flat, do not contain a disintegrant, and are intended for dissolution in the local fluids.

5. Lozenges

Lozenges are slow dissolving compressed tablets that do not contain a disin-tegrant. Some lozenges contain antiseptics (e.g., benzalkonium) or antibiotics for local effects in the mouth. Lozenges are also used for systemic effect, such as those containing vitamin supplements. Lozenges are palatable and organo-leptically appealing by the addition of flavors, sweeteners, and colors.

6. Coated tablets

Most tablets are coated for one or more of the following reasons:

·           To prevent decomposition of drugs sensitive to air (oxygen), light, or humidity

·           To minimize the unpleasant taste of certain drugs that may come dur-ing partial dissolution of the drug in buccal fluids during absorption

·           To improve swallowability and palatability by increasing surface smoothness in the mouth

·           To provide visual appeal and consistency, smooth surface texture, and uniform distribution of color

·           To serve as anticounterfeiting medium by incorporating tracer com-pounds in the coating material

·           To allow containment of highly potent compounds in the core of the tablet and, thus, avoid exposure to personnel handling of the tablets

Coating is not used on buccal, sublingual, chewable, effervescent, or dis-persible tablets to avoid any delay in drug release due to the time required for the rupture or dissolution of the coating material (Table 20.2).

Coating of core (compressed, uncoated) tablets is carried out by load-ing the tablets in a moving, perforated pan supplied with dry hot air and spraying the coating dispersion onto the tablet bed at a rate matched with the rate of evaporation of the solvent. This leads to the deposition of a film of the coating material on the surface of the coated tablets. 

Table 20.2 Characteristics of types of tablet coatings

This process is carried out for a sufficient duration of time to allow uniform and elegant coverage of the entire surface of the tablet by the coating material. The coating material consists of an opacifier (such as fine particle size titanium dioxide), color, plasticizer (such as polyethylene glycol), and a polymer (such as hydroxypropyl methylcellulose [HPMC] or polyvinyl alcohol [PVA]). Typically, about 3% w/w application of the coating material provides com-plete coverage by the formation of a thin film around the tablet. The coated tablets are called film-coated tablets.

Other types of coated tablets are sugarcoated, gelatin-coated (gel caps), and enteric-coated tablets. Sugarcoated tablets are produced by the application of sucrose solution, containing preservatives, colorants, sweeteners, and flavors, to the core with a relatively high (~30% w/w) weight buildup. Film coating has almost completely taken over sugar coating in the pharmaceutical industry because of shorter processing time and the smaller size of the coated tablet. A common example of sugarcoated tablet is the M&Ms, which are consist of a solidified liquid-chocolate center and a hard-candy shell which is a combination of sugar and corn syrup. Characteristics of sugarcoated and film-coated tablets are summarized in Table 20.3.

Table 20.3 Examples of sustained release tablets

Gelatin-coated tablets, commonly known as gelcaps, are capsule-shaped compressed tablets coated with a gelatin layer. These tablets are produced by dipping the core tablets in a solution of gelatin with colors and preser-vatives, followed by drying. This allows the product to be smaller than an equivalent capsule filled with an equivalent amount of powder, and pro-vides an elegant visual appeal. Many over-the-counter (OTC) medications are marketed as gelcaps.

7. Enteric-coated tablets

GI fluid pH increases progressively from acidic to basic from the stomach through the intestines to the colon. The changes in GI pH can be utilized to release a drug at a particular physiological location in the GI tract. In particular, oral solid dosage forms can be coated with a polymer that is insoluble at the acidic stomach pH and soluble at basic intestinal pH. Such polymers are known as enteric polymers and such coatings are termed enteric coatings. Enteric-coated tablets are the tablets coated with enteric polymers. Complete coating of the tablet with these polymers allows the polymer to form a barrier between the core of the tablet and the surround-ing aqueous medium. Thus, the enteric-coated tablet remains insoluble in the low pH environment of the stomach, but dissolves readily on passage into the small intestine with its elevated pH.

Enteric coating is used to minimize irritation of the gastric mucosa by certain drugs and/or protect sensitive drugs against decomposition in the acidic environment of the stomach. For example, aspirin produces less gas-tric bleeding when formulated as enteric-coated sustained-release (SR) tab-lets than conventional immediate release dosage forms.

Commonly used polymers for enteric coating are acid-impermeable poly-mers, such as cellulose acetate phthalate (CAP), HPMC phthalate (HPMCP), polyvinyl acetate phthalate (PVAP), and Eudragits®, which are copolymers of methacrylic acid and methylmethacrylate. Eudragits are available in different grades. The drug release behavior of a Eudragit-coated tablet is controlled through the ratio of methacrylic acid copolymers. For example, the ratio of carboxyl to ester groups is approximately 1:1 in fast dissolving Eudragit L100 and 1:2 in slow dissolving Eudragit S100.

8. Immediate release tablets

Most tablets (discussed earlier) are immediate release (IR) tablets, that is, they make all the drugs available to the dissolution medium immediately on coming in contact with the aqueous medium. The drug dissolves at a rate determined by the composition of the dissolution medium (such as pH) and physicochemical properties of the drug (such as solubility and particle size).

9. Controlled release tablets

In contrast to the IR tablets, certain dosage forms, such as controlled-release (CR) or extended-release (XR) tablets, are designed to control or extend, respectively, the rate at which drug dissolves in the aqueous medium. Thus, CR tablets reduce the rate of drug release to a slow, controlled rate, which is typically zero order. XR tablets, on the other hand, extend the duration of drug release by slowing down the rate of drug release but may not have control on the rate (i.e., may not provide zero-order kinetics of drug release). CR or XR tablets are sometimes also called SR tablets.

CR tablets can reduce dosing frequency, increase patient compliance, and may reduce side effects of certain drugs. The rate-controlling feature of the CR tablets could be either the matrix or the film coating. Coformulating or mixing drugs with water-insoluble polymers prepare matrix tablets. A slow dissolving polymer matrix, such as high molecular weight HPMC, can be used to prepare SR tablets of highly water-soluble drugs. On coming in contact with the aqueous dissolution medium, the core tablet dissolves by surface erosion, and the rate of surface erosion is controlled by the rate of dissolution of the polymer matrix. For example, metformin hydrochloride XR tablets (Glucophage XR tablets®) are matrix tablets.

Membrane-coated SR tablets utilize an insoluble membrane to reduce or control the rate of drug release. The insoluble membrane can be impreg-nated with a soluble polymer to provide pores from which the drug can diffuse out or have a hole for osmotically CR tablets. Often, a combination of IR and SR components is included in a dosage form to provide a loading dose (by the IR component) followed by a slow releasing maintenance dose (by the SR component). The IR and the SR component can form two dif-ferent layers of a bilayer tablet, for example. These components can also be mixed together and packaged in a capsule. The SR and IR components can also be compressed together. For example, Theo-Dur® CR tablet of theoph-ylline consists of two components: (1) a matrix of compressed theophylline crystals and (2) coated theophylline granules embedded in the matrix. In contact with fluids in GI tract, theophylline diffuses slowly through the wall of the free granules, which dissolves with time. After oral administra-tion of Theo-Dur® 300 mg tablets to human subjects, serum theophylline concentrations over 1 mg/ml were maintained over 24 hours.

Examples of commonly used SR drug delivery products are listed in Table 20.3.