enteric coating

An enteric coating is a smart wrapper that stays shut in the acid of the stomach and only opens further down in the intestine. It is like a raincoat that holds in the stomach's harsh acidic conditions and then unbuttons once the surroundings turn from acidic to near-neutral, so the drug is released past the stomach rather than in it.

The trick is a polymer that is insoluble in acid but dissolves above a certain pH, such as cellulose acetate phthalate, hydroxypropyl methylcellulose phthalate or methacrylic acid copolymers. As the coated tablet leaves the acidic stomach and meets the higher pH of the small intestine, the coat dissolves and releases the drug. This is used to protect drugs that acid would destroy (like some enzymes), to shield the stomach lining from irritant drugs (like aspirin), or to deliver a drug specifically to the intestine.

Enteric coating is a form of delayed release, not slow release — once past the stomach the drug usually comes out fairly quickly. A caveat is that gastric emptying varies a lot between people and with food, so the exact time and place of release can be unpredictable, and taking the dose with a heavy meal can delay it considerably.