An enteric coating is a coating put on a pill or capsule so that it doesn't dissolve until it reaches the small intestine. While the coating may make a pill easier to swallow and will mask bitter-tasting medicine, enteric coatings are primarily used because the drug is likely to cause stomach irritation or because its effectiveness might be reduced by stomach acids or enzymes.
Enteric coatings work because they are selectively insoluble substances -- they won't dissolve in the acidic juices of the stomach, but they will when they reach the higher pH of the small intestine.
Most enteric coatings won't dissolve in solutions with a pH lower than 5.5. Commonly-used enteric coatings may be made from:
Most enteric coatings are dissolved in organic solvents such as acetone, methanol, ethanol, isopropyl alcohol, ethyl acetate, methylene chloride, etc. and applied to the tablets or capsules. The coatings might be sprayed on or applied as a chemical vapor, or the tablets might be put in a rotating pan partially filled with the coating. The solvent evaporates, leaving the coating behind.
It is worth noting that some enteric coatings are essentially plastics, and some are phthalates. Certain phthalates have been associated with cancer and birth defects and are suspected endocrine disruptors. There is no data on whether the type and amount of chemicals used as enteric coatings could present a health risk. Such data is not likely to be forthcoming soon.