An enteric coated drug is an oral dosage form wrapped in a pH-responsive polymer film that stays intact in the acidic stomach (pH 1.5 to 3.5) and dissolves only after reaching the higher pH of the small intestine (pH 5.5 to 7.0). The coating protects acid-labile molecules from degradation, protects the gastric mucosa from irritant drugs, and enables site-specific release in the duodenum, jejunum, or colon.
Enteric coating is one of the oldest functional coating technologies in pharmaceutical manufacturing and remains one of the most commonly misunderstood. The film is not a barrier in the mechanical sense. It is a chemically switchable barrier, and almost every field failure traces back to that distinction.
Why bypass the stomach at all?
Three separate formulation problems are solved by the same technology, which is why the term covers such a wide range of products.
1. The drug is destroyed by acid. Proton pump inhibitors such as omeprazole, esomeprazole, and pantoprazole degrade rapidly below pH 4. Without an enteric layer, bioavailability collapses before absorption can begin. Peptides and certain antibiotics such as erythromycin face the same problem.
2. The drug attacks the stomach. Aspirin, diclofenac, naproxen, and other NSAIDs inhibit prostaglandin-mediated mucosal protection and cause direct local irritation. Delaying release below the pylorus reduces gastric erosion, though it does not eliminate systemic NSAID risk.
3. Release must be targeted. Mesalazine for inflammatory bowel disease, pancreatic enzyme replacement, and probiotic formulations all require delivery to a defined intestinal segment. Here the coating is a targeting device rather than a shield.
The chemistry: how a pH-responsive film works
Enteric polymers are weak polyacids. Their backbones carry pendant carboxylic acid groups (from phthalate, succinate, or methacrylic acid units). In gastric fluid, well below the polymer pKa, those groups stay protonated and uncharged, the film remains hydrophobic, and water uptake is minimal.
As the dosage form moves into the duodenum and local pH rises above the polymer’s threshold, the carboxyl groups ionize. The polymer becomes anionic, hydrates, swells, and dissolves. The threshold is not a sharp switch but a narrow band, typically spanning 0.5 to 1.0 pH units, which is why polymer selection is a matter of matching a dissolution profile to a target intestinal region rather than picking a single number.
Enteric polymer selection
| Polymer | Dissolution threshold | Notes |
| Polyvinyl acetate phthalate (PVAP) | ~pH 5.0 | Fast duodenal release, phthalate ester hydrolysis risk on storage |
| Hypromellose phthalate HP-50 | ~pH 5.0 | Good film former, moisture sensitive |
| Hypromellose phthalate HP-55 | ~pH 5.5 | Widely used for proton pump inhibitors |
| Methacrylic acid copolymer Type C (L 30 D-55 / L100-55) | ~pH 5.5 | Aqueous dispersion available, low-temperature processing |
| Cellulose acetate phthalate (CAP) | ~pH 6.0 | Long-established, brittle, prone to hydrolysis and acetic odour |
| Methacrylic acid copolymer Type A (L100) | ~pH 6.0 | Jejunal targeting |
| HPMCAS (L, M, H grades) | pH 5.5 / 6.0 / 6.8 | Non-phthalate, excellent stability, also used as an ASD carrier |
| Methacrylic acid copolymer Type B (S100) | ~pH 7.0 | Ileal and colonic targeting |
| Shellac | ~pH 7.2 | Natural origin, polymerises on ageing, dissolution slows over shelf life |
Two selection notes that separate a robust product from a fragile one. First, phthalate-based polymers hydrolyse over time, releasing free phthalic acid that can react with amine-containing APIs. Non-phthalate options (HPMCAS, methacrylic acid copolymers) avoid this entirely. Second, all enteric films need a plasticiser (triethyl citrate, dibutyl sebacate, or PEG at roughly 10 to 25 percent of polymer weight) to drop the glass transition temperature below processing and storage conditions. Under-plasticised films crack.
The coating process
Enteric coating is almost always a spray application in a perforated side-vented pan (tablets) or a Wurster fluid bed column (pellets, minitablets, and granules for capsule filling).
Sub-coating. A thin seal coat of HPMC or PVA (1 to 2 percent weight gain) is applied first for two reasons: it isolates alkaline or amine-bearing APIs from the acidic enteric polymer, and it smooths surface roughness so the functional layer builds evenly.
Functional layer weight gain. This is the single most important process variable. Typical targets are 6 to 10 percent for standard tablets, 8 to 12 percent for tablets with deep debossing or sharp edges, and 20 to 40 percent for small pellets, where the surface-area-to-volume ratio is far higher. Under-coating is the most common root cause of acid stage failure.
Critical process parameters. Spray rate, atomisation air pressure, inlet air temperature, and product bed temperature are interdependent. Aqueous dispersions of methacrylic acid copolymers run at a bed temperature of roughly 28 to 32 degrees Celsius. Too cool and the droplets do not coalesce, leaving a porous discontinuous film. Too hot and the film dries before levelling, producing an orange-peel surface with pinholes.
Curing. Aqueous latex dispersions require a post-coating hold (commonly 40 degrees Celsius for 2 to 24 hours) to complete polymer particle coalescence. Skipping the cure step gives a coating that passes release testing and fails at six-month stability.
Common failure modes
Acid stage failure in USP <711>. Delayed release testing runs 2 hours in 0.1 N HCl (not more than 10 percent released) followed by a buffer stage at pH 6.8. Failures usually mean insufficient weight gain, poor coating uniformity, or an incomplete cure.
Edge and logo erosion. Tablet edges and debossed characters receive thinner film and take the most attrition in the pan. Deep or sharp-cornered debossing is a coating liability. Bisect lines are worse: they create a channel where the film thins and cracks under compression from tumbling.
Coating bridging. The film pulls away from a debossed character during drying, forming a void that later ruptures. Increasing plasticiser level and reducing drying rate both help.
Ageing. Shellac continues to polymerise on storage, and dissolution slows progressively. Phthalate polymers hydrolyse under humidity. Both effects mean a coating that meets specification at release may not at month 24.
Physiological variability. Even a perfect coating cannot control gastric emptying. Fasted transit through the stomach may take 15 minutes, fed transit two hours or more. Duodenal pH also varies between individuals and with disease state, which is why a pH 5.5 polymer with a wider dissolution band is often the safer commercial choice over a sharp pH 7.0 threshold.
Frequently asked questions
Can enteric coated tablets be crushed or split? No. Any breach of the film exposes the API to gastric acid and defeats the entire design. This is a routine and clinically significant medication error.
What is the difference between enteric coated and delayed release? Enteric coating is one mechanism for achieving delayed release. Delayed release is the broader functional category and can also be achieved by time-dependent or enzyme-triggered systems.
Do enteric coated pills work more slowly? Yes. Onset is typically delayed by 1 to 3 hours because release cannot begin until gastric emptying is complete. Enteric formulations are not appropriate where rapid onset is required.
Is enteric coating the same as sugar or film coating? No. Sugar and standard film coats are non-functional, applied for taste masking, appearance, and handling. Enteric coats are functional and are subject to dissolution specifications.
Disclaimer:
This article is for informational and educational purposes for life sciences professionals. It is not medical advice, formulation guidance, or a regulatory reference. Technical values are indicative and must be verified against current pharmacopoeial and regulatory requirements for your product and jurisdiction. Patients should consult a qualified healthcare professional regarding any medication.