Enteric (Functional) Coating

An enteric coating is a polymer film applied to a tablet, pellet or capsule that is insoluble in the acidic environment of the stomach (pH ~1.2 fasted, up to pH 5 fed) but dissolves in the more neutral or alkaline environment of the small intestine (pH ~5.5 to 7.4). Functionally, it delays the release of the active pharmaceutical ingredient (API) until the dosage form has cleared the stomach — typically 30 minutes to a few hours after ingestion.

The term 'enteric-coated' is sometimes written EC and appears in product names (e.g. 'aspirin EC', 'naproxen DR'). Pharmacopoeias refer to enteric-coated products as 'gastro-resistant' (Ph. Eur.) or 'delayed-release' (USP) — the three terms are functionally interchangeable for oral solid dosage forms.

There are four main reasons a formulator chooses an enteric coating over a conventional immediate-release film.

The list below covers widely prescribed enteric-coated (delayed-release / gastro-resistant) products. The 'EC', 'DR' or 'gastro-resistant' suffix in the product name is the usual market indicator.

All enteric polymers carry weakly acidic functional groups (carboxylic acid, phthalate, succinate) that remain protonated and insoluble at low pH but ionise and dissolve as the surrounding pH rises above the polymer's threshold.

Polymer choice is driven by the desired release site (duodenum vs jejunum vs ileum vs colon), the API stability, the coating method (aqueous vs organic solvent), and regulatory acceptability in the target markets.

Enteric coats are almost always applied in a perforated pan coater (Glatt, O'Hara, Vector) or a fluid-bed Wurster column. The polymer is sprayed as an aqueous dispersion (preferred — no organic solvent residue) or an organic solvent solution, with simultaneous drying to build a continuous film.

1. Core tablets / pellets are warmed in the coating pan to the target product temperature (typically 30–40 °C for aqueous methacrylates).
2. Sub-coat (optional) — an HPMC or PVA seal coat to protect the core from coating moisture and provide a smooth substrate.
3. Enteric polymer dispersion is sprayed at a controlled rate, with atomising air, drying air and pan speed all controlled inline.
4. Coating builds to a target weight gain — typically 6–12% w/w for full gastric resistance; lower for re-coats, higher for high-acid-sensitivity APIs.
5. Curing step (15–30 min at slightly elevated temperature) lets the latex coalesce into a continuous film. Omitting cure is the #1 cause of failed acid-stage dissolution.
6. Top-coat (optional) — colour, brand or moisture-barrier layer applied after the enteric polymer.

Enteric-coated (delayed-release) products are tested by a two-stage dissolution method per USP <711> and Ph. Eur. 2.9.3.

• Disintegration is tested separately (USP <701>) — no disintegration in acid, full disintegration in buffer.
• Residual solvents (acetone, IPA) tested by GC headspace if organic coating used (ICH Q3C).
• Coating weight gain logged per batch and trended for CPV.
• Microbial limits (USP <61> / <62>) tested per the monograph or in-house spec.

• Skipping or under-curing the latex film — coalescence is incomplete, acid permeates and the unit fails acid-stage dissolution.
• Under-application of polymer (weight gain < 6%) — gastric resistance is incomplete in the fed state (which buffers to pH 4–5).
• Over-spray-rate causing tablet sticking and twinning — coating thickness varies and acid resistance becomes batch-variable.
• Using the wrong polymer grade for the target release site (e.g. L100-55 where S100 was needed for colonic delivery).
• Ignoring plasticiser level — under-plasticised films crack on storage and lose gastric resistance.
• No moisture-barrier sub-coat with hygroscopic APIs — the core picks up coating water and degrades.
• Patient counselling failure — pharmacy must instruct 'swallow whole, do not crush' or the coating is defeated.