If the active ingredient of a tablet is sensitive to acid, or is irritant to the stomach lining, an enteric coating can be used, which is resistant to stomach acid and dissolves in the high pH of the intestines. Enteric coatings are also used for medicines that can be negatively affected by taking a long time to reach the small intestine where they are absorbed. Coatings are often chosen to control the rate of dissolution of the drug in the gastro-intestinal tract. Some drugs will be absorbed better at different points in the digestive system. If the highest percentage of absorption of a drug takes place in the stomach, a coating that dissolves quickly and easily in acid will be selected. If the rate of absorption is best in the large intestine or colon, then a coating that is acid resistant and dissolves slowly would be used to ensure it reached that point before dispersing. The area of the gastro-intestinal tract with the best absorption for any particular drug is usually determined by clinical trials.
This is the last stage in tablet formulation and it is done to protect the tablet from temperature and humidity constraints. It is also done to mask the taste, give it special characteristics, distinction to the product, and prevent inadvertent contact with the drug substance. The most common forms of tablet coating are sugar coating and film coating.
Coating is also performed for the following reasons:
Controlling site of drug release
Providing controlled, continuous release or reduce the frequency of drug dosing
Maintaining physical or chemical drug integrity
Enhancing product acceptance and appearance
Sugar coating is done by rolling the tablets in heavy syrup, in a similar process to candy making. It is done to give tablets an attractive appearance and to make pill-taking less unpleasant. However the process is tedious and time-consuming and it requires the expertise of highly skilled technician. It also adds a substantial amount of weight to the tablet which can create some problems in packaging and distribution.
In comparison to sugar coating, film coating is more durable, less bulky, and less time consuming. But it creates more difficulty in hiding tablet appearance. The purpose of this coating is to prevent dissolution of the tablet in the stomach, where the stomach acid may degrade the active ingredient, or where the time of passage may compromise its effectiveness, in favor of dissolution in the small intestine, where the active principle is better absorbed.this website http://www.tabletsdosageform.blogspot.com/ is dedicated for educting pharmaceuticle students
tablets dosage form advantages and disadvantages