Solid Dosage Forms: Tablets

h. Lozenges or Troches

These are disc-shaped solid preparations containing medicinal agents and generally a flavouring substance in a hard candy or sugar base. They are intended to be slowly dissolved in the oral cavity, usually for local effects.

Examples include Strepsils Dry Cough Lozenges – Dextromethorphan Hydrobromide 5mg, Dichlorobenzyl alcohol 1.2mg, Amylmetacresol 0.6mg (Reckitt Benckiser), Dequadine – Dequalinium chloride BP 250mcg (Evans Medical PLC), Dr Meyer Coflin cough lozenges (Meyer Organics PVT Ltd), Cofta – Ammonium chloride/ Ipecacuanha tablet (Evans Medical PLC) etc.

i. Tablet Triturates

Tablet triturates are small, usually cylindrical, moulded, or compressed tablets containing small amounts of usually potent drugs mixed with a combination of sucrose and lactose or any suitable diluent. They are prepared from moist material, using a triturate mould that gives them the shape of cut sections of a cylinder.

Since tablet triturates must completely and rapidly dissolve in water, only a minimal amount of pressure is applied during their manufacture. One of the problems encountered during the manufacture of this tablet type is the failure to find a lubricant that is completely water-soluble. A typical example of tablet triturate is NTG tablets.

j. Hypodermic Tablets

Hypodermic tablets are soft, readily soluble tablets that were originally used by physicians in extemporaneous preparation of parenteral solutions. These tablets are dissolved in a suitable vehicle (water for injections) and administered by parenteral route.

Hypodermic tablets are no longer used in most countries due to the difficulty in achieving sterility. Also, the availability of stable parenteral solutions and prefabricated injectable products, some in disposable syringes have also discouraged their use in recent times. e.g., Dilaudid – Dihydromorphinone HCl (Bilhuber Knoll Corp.).

k. Dispensing Tablets

Dispensing tablets also referred to as compounding tablets are tablets supplied primarily as a convenience for extemporaneous compounding. These tablets contain large amounts of highly potent APIs, and thus are used by a pharmacist to compound prescriptions that can be incorporated readily into powders and liquids, thus, circumventing the necessity to weigh small quantities of these potent drug substances.

Dispensing tablets are no longer in use and had the dangerous potential of being inadvertently dispensed as such to patients. Examples include silver potentiate, bichloride of mercury merbromin and quaternary ammonium compounds.

l. Gelatin-Coated Tablets

Gelatin-coated tablets are compressed tablets coated with either one or two-toned colour gelatin. The gelatin coating impacts the same general characteristics as sugar coating and film coating with the added advantage of improving the stability of photosensitive APIs.

The gelatin coating also facilitates swallowing, enables custom branding,  and prevents counterfeit since they are more tamper-evident than unsealed capsules. Gelatin-coated tablets are also ideal for double-blind clinical studies, or for drug substances that can irritate the oesophagal mucosa when they are incorporated in an immediate-release tablet such as bisphosphonates.

Example of gelatin-coated tablets includes gelatin-coated hydrochlorothiazide tablet (Qualitest Pharmaceuticals), Tylenol Cold Multi-Symptom Daytime (McNeil Consumer) etc.

m. Multiple Compressed Tablets/ Multi-compressed Tablets

Multiple compressed tablets, also called multi-compressed tablets are tablets that are composed of two or more layers. These tablets are prepared by subjecting the fill material to more than one compression cycle.

The result may be a multiple-layer tablet or a tablet within a tablet, the inner tablet being the core and the outer portion being the shell. This process is best used when separation of active ingredients is needed for stability purposes or if the mixing process is inadequate to guarantee uniform distribution of two or more active pharmaceutical ingredients.

Multiple compressed tablets can also be used when there is a need to mask the bitter taste of a drug substance or where the drug substance in question is irritant to the stomach. There are three subclasses of multiple compressed tablets and they include compression coated tablets, layered tablets and inlay tablets.

i. Compression Coated Tablets

Compression coated tablets also referred to as dry-coated tablets or press-coated tablets, are tablets with two parts; internal core and surrounding coat. These tablets are prepared by feeding previously compressed tablets into a special tablet press (e.g., Manesty Drycota) and compressing another granulation layer around a preformed tablet core.

Compression coated tablets have all the advantages of compressed tablets (i.e., slotting, monogramming, speed of disintegration) while retaining the attributes of sugarcoated tablets in masking the taste of the drug substance in the core tablets.

These tablets can also be used to separate incompatible drug substances (one in the core and the other in the coat); in addition, they can provide a means of giving an enteric coating to the core tablets.

ii. Layered Tablets

They are tablets composed of two or more layers of ingredients. Layered tablets are prepared by compressing additional tablet granulation on a previously compressed granulation to form two-layered or three-layered tablets, depending on the number of separate fills. Each layer may contain a different medicinal agent, separated for reasons of physical or chemical incompatibility, staged drug release, or simply the unique appearance of the layered tablet.

Unlike conventional tablets where we have a single piece of substance moulded to shape, layered tablets have the appearance of a sandwich because the edges of each layer are exposed.

iii. Inlay Tablets

Inlay tablets, popularly known as dot, or bull’s-eye tablets are variation of compressed tablets with a partially surrounded core. Instead of the tablet core being completely surrounded by the coating, its top surface is completely exposed.

Inlay tablets are prepared by feeding previously compressed tablets into a prefilled die cavity of Stokes, Colton, or Kilian machines. When compressed, some of the coating material is displaced to form the sides.  With a yellow core and a white coating, Inlay tablets resemble a fried egg.

Inlay tablets can be useful in sustained-release preparations to reduce the size and weight of the tablet. A typical example is a European preparation containing 25 mg of hydrochlorothiazide in the bull’s-eye and 600 mg of potassium chloride in the outside portion.

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