Coated tablets are defined as “tablets covered with one or more layers of mixture of various substances such as: Natural or Synthetic Resins, Gum, Sugar, Waxes, Authorized Colouring Materialand Some Times Flavoring Material etc.

Coating of the tablet was done for the following reason-

1. To mask the unpleasant taste and odour.
2. To improve the appearance of the tablets
3. To prevent the medicaments from atmospheric effects.
4. To control the site of action of drugs.
5. To produce the sustained released product.

Types of tablet coating:

1. Sugar coating
2. Enteric coating
3. Film coating
4. Gelatin coated tablet
5. Compressed coating Tablet

1)   Sugar Coating: Sugar coating is usually soluble in water, causing it to dissolve quickly when exposed to any liquid medium, for example, gastrointestinal fluid. The major advantage of sugar coating is that it eliminates unpleasant or bitter taste, odour, and makes it easier to swallow.

2)   Enteric coating: Enteric coating tablets have a cellulose coating or other materials that prevent their degradation in stomach and make them usable in the intestine. Enteric coating is a process that is particularly prepared to work the tablet in the alkaline pH of the intestine rather than the acidic pH of the stomach.

3)   Film coating: Enteric coating tablets have a cellulose coating or other materials that prevent their degradation in stomach and make them usable in the intestine. Enteric coating is a process that is particularly prepared to work the tablet in the alkaline pH of the intestine rather than the acidic pH of the stomach.

4)   Gelatin coated tablet: Gelatin coating tablets are made by spraying less bloom solution of gelatin and water onto pre-coated tablets, the gelatin coating provides a lower coefficient of friction and consequently increased slippery and swallowing ability for tablets without thickness and stickiness. It is suitable for both wet granulation and direct compression. The pharmaceutical manufacturers also broadly used the empty gelatin capsules to encapsulate dosage forms of various drugs.

5)   Compression coating:Compression coating is of the significant dry coating method that does not need either heat or solvent to perform the coating. It is used to protect the active drug component (API) from the environment and is also used to release modified medicine. The important area where compression coating is widely used is site-specific drug delivery, especially colon-specific drug delivery. Compression coating is a system in which the whole surface of an inner core is surrounded by a coat. It is widely used for particularly colon-specific drug delivery and site-specific drug delivery.

The uncoated tablet is a single layer or multi-layer of formulation that contains the drugs or medicines and excipients simultaneously without any coating. In general, it is a single compression of granules or multi-layer tablets consisting of parallel layers prepared by compression of granules of various compositions. These types of pills do not require additional processing after compression. The majority of uncoated tablets are made in such a way any substances added are not intended expressly to alter the release rate or dissolution rate of their active ingredients. Soluble tablets, dispersible tablets, and effervescent tablets are some types of uncoated tablets.

1. Sustained release Tablet

Sustained release dosage form is defined as well characterized and reproducible dosage form, which is designed to control drug release profile at a specified rate to achieve desired drug concentration either in blood plasma or at target site.This system will provide actual therapeutic control that would be temporal (time related), spatial (site related) or both

Advantages of sustained drug delivery system:

• Reduced see-saw fluctuations.
• Total amount of dose decreases.
• Improved patient compliance.
• Increased safety of drugs [15-19].

Disadvantages of sustained drug delivery system:

• Chances of dose dumping.
• Dose retrieval is difficult.
• High cost of formulation.
• Need for additional patient education.
• Reduced potential for accurate dose adjustment

2. Extended release tablet

The drug delivery system that allows at least two folds reduction in dosage frequency as compared to that drug presented as an immediate release system. They also differ from immediate release tablets which release content within few minutes of ingestion ER tablets slowly release drug into the body over a period of time, usually 12 hours or 24 hours and are only taken once or twice a day. They are specially designed to release a pre-designated amount of medication throughout the day.

Advantages

• Less frequent dosing
• Fewer side effects
• Fewer fluctuations in blood levels,
• Complete absorption
• Potential to improved patient compliance and convenience
• Increased stability,improved bioavailability, providing special effects, etc.
• The drug release rates are affected by presence of food in GIT

Disadvantages

• Slower onset of action
• Comparatively high cost
• The size of ER tablet may cause difficulties in ingestion or transit through gut.
• Sometimes the target tissue is exposed to constant amount of drugover extended period resulting in drug tolerance
• Air or moisture can change the effectiveness of ER tablets, so it is important to store them in sealed bottles and in a dry place.

3. Fast Dissolving Tablets

• Fast dissolving tablets (FDT) are solid dosage forms designed to dissolve in saliva remarkably faster, within a few seconds (usually less than 60 sec), and those are real fast dissolving tablets.
• FDTS disintegrate rapidly in saliva without the need to take water and are of commonly used for the pediatric and geriatric patients who have difficulties such as fear of choking while swallowing them with water or have difficulties in chewing them; sometimes very elderly patients of depression may not be able to swallow conventional tablets.
• FDTS can also be known as orally disintegrating or dispersible tablets (ODTS), mouth melting tablets (MMTs) or mouth dissolving tablets (MDTs), immediate release (IR) tablets.
• Mouth dissolving tablets are formulated by use of super disintegrants like Croscarmellose sodium, sodium starch glycolate and crospovidone or by maximizing pore structure of the Tablets by freeze drying and vacuum drying.

Advantage:-

• No need of water to swallow the Tablet
• Easy administration to pediatric, elderly and mentally disabled patients.
• Faster dissolution and absorption of the drug offering rapid onset action
• Increased bioavailability
• Reduce first pass metabolism

Disadvantage:-

• FDTS being highly porous, soft molded metrics or compressed with low compressional pressure are friable and brittle which make them difficult to handle
• Several of the FDT are hygroscopic and thus cannot maintain its physical integrity under normal humidity condition and thus requires specialized package.
• Dryness of the mouth due to decreased saliva production may hamper dissolution, rate of absorption from the saliva solution and overall bioavailability.

4. Multilayered Tablet

Multilayer tablet consists of layers of drug with different release rate, having ability to prevent drug-excipient incompatibility. It provides multiple release kinetics profile in single delivery system of one or more drugs. In this immediate release and then sustained release of drug is designed as control system. Immediate release layer is designed with disintegrating monolithic matrix in order to achieve initial peak and sustained release layer is designed with erodible monolithic matrix to deliver the drug as later part to maintain the drug plasma concentration. The mechanism of drug release from multilayer tablets.

Advantages of drugs

1. Cost is lower compared to all other oral dosage form.
2. Greatest chemical and microbial stability over all oral dosage form.
3. Unpleasant odour and bitter taste can be masked by coating technique.
4. They are unit dosage form and offer the greatest capabilities of all oral dosage form for the greatest dose precision and the least content variability.
5. Easy to swallowing with least tendency for hang-up.
6. The tablet can be easily used for combination therapy.
7. In case of drugs having a low half-life, each of the two layers of the tablet respectively content a loading dose and maintenance dose of the same and thus increase the bioavailability of the drug.
8. Improved patient compliance.

Disadvantage of Multilayered Tablet:

1. Some drugs resist compression into dense compacts, owing to amorphous nature, low density character.
2. Bitter testing drugs, drugs with an objection able odour or drugs that are sensitive to oxygen may require encapsulation or coating.
3. Difficult to swallow in case of children and unconscious patients.
4. Drugs with poor wetting, slow dissolution properties, optimum absorption high in GIT may be difficult to formulate or manufacture as a tablet that will still provide adequate or full drug bioavailability.
5. Lack of sufficient bonding and adhesion at the interface between the adjacent compacted layers which is often the result of an interfacial crack and layer separation.
6. Other challenges during development include establishing the order of layer sequence layers, first layer tamping force, and cross contamination between layers.
7. The physician has a less flexibility on adjusting the dose regimens.