**Abstract**

Oral solid dosage forms hold a predominant position in the drug delivery system. Tablets are the most widely used and convenient dosage form. Due to their ease of manufacturing, the minimum cost of production, easy handling and storage, and better stability, tablets are most preferred. Patients who are prescribed more than one drug are in a situation to consume multiple tablets. To minimize the counts, one or more drugs are cast into layers to form a single tablet, thus called layered tablets. Layered tablets tend to improve patient compliance and reduce the cost of production by half. Layers can be of multiple drugs or the same drug at different doses or drugs with release enhancers or drugs with fillers. Layered tablets hold a greater potential with better patient outcomes as well as stay production-friendly.

**Keywords:** Oral solid dosage form, layered tablets, compliance, release enhancers

### **1. Introduction**

A particular route of administration is conferred to a drug therapy based on its intended site of action, physicochemical properties, targeting aspects, the convenience of administration, stability, duration and onset of action, and many others. All these parameters help in building a stable, efficient, and therapeutically sound product. The most commonly used routes of administration include oral, IV, IM, and transdermal.

Since the era of drug delivery, the oral dosage is retaining an unbeatable position in drug administration. A drug delivery system aims at improving the efficiency of the treatment and various parameters like handling, administration, storage, etc. The oral route of drug administration is one of the oldest choices and has been consistently dominating the world of drug delivery. It comes with enormous merits like improved patient compliance, a simple manufacturing process, less complex requirements, easy storage and handling, and so on. Despite hindrances like first-pass metabolism, slow onset of action compared to IV/IM, and recent advances in novel drug delivery, the oral solid dosage form has still not lost its dominance.

The oral route is the most widely accepted and marketed route of administration. Besides convenience, it gives the advantage of enhanced absorption. The gastrointestinal tract has a larger surface area conferring to increased absorption of drugs. The intestinal epithelial wall is composed of villi, a micro- erective structure, that

increases the absorptive surface area in the gastrointestinal tract up to 300–400 m2 [1]. Pharmaceutical active ingredients are mostly weak acids or weak bases. The absorption of drugs is based on pH and dissociation constant. Drugs exhibit various degrees of absorption at differing pH. GIT offers a wide range of pH from highly acidic to highly basic nature (0.8–8). This enables absorption of both acidic and basic drugs. The stomach is a primary organ for the absorption of acidic drugs. The intestine has a grading basic pH enhancing the absorption of basic drugs [2, 3].

The oral dosage form denotes systems administered by the oral route. Being the most anticipated route, many formulations are available in the market. They can be solids, liquids, and semi-solids. One of the major factors to be considered for developing a formulation is its pharmaceutical stability. Solids showcase high mechanical, microbial, and chemical stability. An added advantage of oral solids is they do not require sterile manufacturing needs. No much sophistication is needed for manufacturing. Simple instruments are sufficient to produce large quantities of oral solids.

Commonly seen oral solids are tablets, capsules, and pellets. Oral solids are non-invasive and stand out due to its higher compliance and accepted for long term therapy. Production cost of oral solids are lower with retail price remains affordable too. Mechanical strength makes it easier to handle, transport and store. They can also provide modified release of drug enabling sustained, controlled and immediate release.

Despite all these advantages, oral solids bear some disadvantages. Solids are not a preferred dosage form for geriatric and pediatric patients. Dysphagic patients find it difficult to swallow solids and it is not an option for unconscious patient. Even with immediate release mechanism, it takes a lag time to disintegrate, dissolve and reach systemic circulation. It may take some to initiate onset of action, making it not an option for emergency conditions. One unavoidable hinderance is first pass metabolism. GIT is a high degradative pathway conferring to presence to enzymes, acid secretions and altering transit time. Recent advancements like oro-dispersible tablets and sublingual tablets are available, but still oral solids are not comparable to parenteral.

### **2. Tablets**

Tablets are popularly denoted as unit dosage form is an which contains active ingredient(s) and excipients, compressed into a compact solid. It is available in various shapes like, circular, cylindrical, triangular. They are mostly circular, with convex ends and blunt edges. Tablets are manufactured by compressing the actives and other ingredients by use of punches and dies. Tablets may carry break line, break marks and symbols for breaking and identification. They are usually swallowed with liquids, may be chewable, oral disintegrating, etc.….

Tablets are unit dosage form that offers accurate dosing at an affordable price. With increased consumption, simple large-scale production makes it more feasible. Other advantages include easy and cost-effective handling, and ease of identification, as tablets come in various shapes, sizes and colors.

Tableting may be affected by due to the poor compressibility and flow properties of powders which is a major compression parameter. The minimum dose also plays a role in compression. Unacceptable taste and odour make it difficult to devise formulation. Physical instability is a serious drawback that can affect the formulation which could be overcome by encapsulation.

In order to classify the tablets representatively, a schematic flow chart is shown in **Figure 1**.

**4**

**Figure 1.** *Flow chart of types of tablets.*
