Stomach inflammation, known medically as gastritis, is most commonly caused by a bacterial infection, overuse of pain medications, or heavy alcohol consumption. Globally, over 38 million people were living with gastritis in 2021, a 57% increase from 1990. The condition ranges from a brief flare-up that heals on its own to a long-lasting process that, left unchecked, can damage the stomach lining enough to raise the risk of ulcers or cancer.
H. pylori: The Most Common Cause Worldwide
A spiral-shaped bacterium called Helicobacter pylori infects up to half the world’s population, with higher rates in developing countries. Most people pick it up in childhood through contaminated food, water, or close contact with an infected person. Many carriers never develop symptoms, but in a significant number, the infection triggers chronic stomach inflammation that can persist for decades.
H. pylori survives the stomach’s harsh acid environment by producing an enzyme that neutralizes acid in its immediate surroundings. It then burrows into the mucus layer that protects the stomach wall, attaches to the surface cells, and releases toxins that directly damage tissue. The immune system responds by flooding the area with inflammatory cells, but this response often fails to clear the infection and instead causes collateral damage to the stomach lining itself. Over time, this cycle of bacterial damage and immune overreaction can erode the protective barrier, leading to gastritis, ulcers, or both.
Pain Medications That Weaken the Stomach Lining
Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, aspirin, and naproxen are the second most common cause of gastritis. These medications work by blocking enzymes called COX-1 and COX-2, which produce compounds involved in pain and inflammation. The problem is that those same compounds also maintain the stomach’s protective mucus layer, regulate blood flow to the lining, and help repair minor damage.
When NSAIDs suppress these protective compounds, the stomach becomes vulnerable on multiple fronts. The stomach begins contracting more forcefully, which increases the permeability of the lining. Blood flow to the mucosa decreases. Immune cells called neutrophils infiltrate the tissue, and harmful oxygen molecules accumulate. The body tries to compensate: when one enzyme pathway is blocked, the other ramps up to keep producing protective compounds. But when both pathways are suppressed simultaneously, which most NSAIDs do, the stomach’s defenses collapse. This is why taking NSAIDs occasionally rarely causes problems, but daily use over weeks or months frequently leads to inflammation, erosions, or ulcers.
Alcohol and Other Chemical Irritants
Heavy or binge drinking is one of the most common triggers for acute gastritis. Ethanol damages the stomach lining through a straightforward chain of events: it ramps up the production of gastric acid by increasing the activity of acid-pumping proteins in stomach cells, while simultaneously stripping away the mucus barrier that normally keeps that acid from touching the lining. The excess acid then damages the tissue underneath, including tiny blood vessels in the stomach wall, causing localized bleeding and microulcers. Biopsies of alcohol-damaged stomachs show clear loss of surface cells, distortion of the tissue structure, and invasion of inflammatory cells into the deeper layers.
Other chemical irritants follow similar patterns. Caustic substances, certain supplements taken on an empty stomach (particularly iron and potassium), and chronic exposure to bile from the small intestine can all erode the mucus barrier and trigger inflammation.
Bile Reflux Into the Stomach
Bile is produced by the liver to help digest fats in the small intestine. Normally, a muscular valve called the pylorus keeps bile from flowing backward into the stomach. When this valve doesn’t close properly, bile washes into the stomach and irritates the lining.
Several conditions increase the risk. Gallbladder disease, including gallstones or polyps, is a common contributor. Weight-loss surgery and gallbladder removal can both alter the anatomy enough to allow reflux. Obesity (a BMI over 30) is independently associated with bile reflux. Smoking and heavy drinking can also loosen the pyloric valve, compounding the problem. Unlike acid reflux, bile reflux doesn’t respond to acid-reducing medications, which makes it a distinct and sometimes frustrating cause of persistent stomach inflammation.
Autoimmune Gastritis
In autoimmune gastritis, the immune system mistakenly attacks the stomach’s acid-producing cells, called parietal cells. These cells contain a protein that pumps acid into the stomach, and the immune system generates antibodies that target this protein specifically. Several types of immune cells participate in the destruction: some kill parietal cells directly through toxic pathways, while others produce signaling molecules that sustain the attack in a self-reinforcing loop.
As parietal cells are destroyed, the stomach loses its ability to produce acid and a substance called intrinsic factor, which is essential for absorbing vitamin B12. This is why autoimmune gastritis often leads to B12 deficiency and, eventually, a condition called pernicious anemia. The inflammation concentrates in the upper portion of the stomach (the body and fundus) and tends to progress slowly over years. It’s more common in people who already have other autoimmune conditions, such as thyroid disease or type 1 diabetes.
Allergic and Eosinophilic Gastritis
A less common but increasingly recognized cause involves eosinophils, a type of white blood cell normally involved in fighting parasites and mediating allergic responses. In eosinophilic gastritis, these cells accumulate in the stomach wall in abnormal numbers, causing inflammation and tissue damage.
Between 45% and 63% of people with this condition also have other allergic disorders like asthma, eczema, or rhinitis, and about 64% have a family history of allergic disease. In roughly half of cases, allergy testing identifies a food trigger. The underlying mechanism involves both immediate allergic reactions and slower, delayed immune responses. Signaling molecules recruit eosinophils to the stomach lining and keep them active there, perpetuating the inflammation. Parasitic infections and certain medications can also act as triggers, though in many cases no specific cause is found.
Stress-Related Stomach Damage
Severe physical stress, the kind experienced during a stay in intensive care, major surgery, or extensive burns, can cause rapid stomach inflammation within hours. This is distinct from everyday psychological stress, which plays a less direct role.
The mechanism centers on blood flow. During a critical illness or severe injury, the body diverts blood away from the digestive system to support vital organs. This reduced blood supply starves the stomach lining of oxygen and nutrients, weakening its ability to produce the protective mucus layer. Without that barrier, even normal levels of stomach acid can damage the exposed tissue. Notably, acid production in these patients is usually normal or even decreased. The problem isn’t too much acid; it’s a stomach lining too weakened to defend against any acid at all. In older adults, existing narrowing of blood vessels from atherosclerosis can make this process worse.
When Gastritis Becomes a Long-Term Risk
Most acute gastritis, whether from a night of heavy drinking or a short course of NSAIDs, resolves once the irritant is removed. Chronic gastritis is different. When inflammation persists for months or years, the stomach lining can undergo a change called atrophy, where the normal glandular tissue is gradually replaced by scar-like or intestinal-type tissue.
The stakes of this progression are real. In a Korean study tracking over 5,500 people, gastric cancer or precancerous changes developed in 3.2% of those with atrophic gastritis, compared to just 0.1% of those with a normal stomach lining. The risk climbed steeply with severity: 1.6% for mild atrophy, 5.2% for moderate, and 12% for severe. The annual cancer incidence for people with atrophic gastritis was about 0.1%, rising to 0.25% once intestinal-type changes appeared. These numbers aren’t meant to alarm, but they explain why gastroenterologists take chronic gastritis seriously and monitor it over time, particularly when H. pylori or autoimmune disease is involved.
Doctors assess the severity and type of gastritis using a standardized system that combines tissue samples from at least five specific locations in the stomach with the patient’s symptoms and lab results. The degree of inflammation, the presence of H. pylori, any loss of normal glands, and any intestinal-type tissue changes are each scored on a four-point scale. This staging helps predict which patients are at higher risk and need closer follow-up.