Most enteric coatings work by presenting a surface that is stable at the highly acidic pH found in the stomach, but breaks down rapidly at a less acidic (relatively more basic) pH. For example, they will not dissolve in the gastric acids of the stomach (pH ~3), but they will in the alkaline (pH 7-9) environment present in the small intestine. Materials used for enteric coatings include fatty acids, waxes, shellac, plastics, and plant fibers.
Drugs that have an irritant effect on the stomach, such as aspirin, can be coated with a substance that will dissolve only in the small intestine. However, it has been shown that this may lead to incomplete inhibition of platelets. Likewise, certain groups of proton pump inhibitors (esomeprazole, omeprazole, pantoprazole and all grouped azoles) are acid-activated. For such types of drugs, enteric coating added to the formulation tends to avoid activation in the mouth and esophagus.
Recently, some companies have begun to apply enteric coatings to fish oil (omega-3 fatty acids) supplements. The coating prevents the fish oil capsules from being digested in the stomach, which has been known to cause a fishy reflux.
Sometimes the abbreviation "EC" is added beside the name of the drug to indicate that it has an enteric coating.
Composition of coatings
- Methyl acrylate-methacrylic acid copolymers
- Cellulose acetate succinate
- Hydroxy propyl methyl cellulose phthalate
- hydroxy propyl methyl cellulose acetate succinate (hypromellose acetate succinate)
- polyvinyl acetate phthalate (PVAP)
- methyl methacrylate-methacrylic acid copolymers
- Cellulose acetate trimellitate
- Sodium alginate