Acid reflux happens when stomach acid flows backward into the esophagus, the tube connecting your mouth to your stomach. A ring of muscle at the bottom of the esophagus normally keeps acid where it belongs, but when that muscle relaxes at the wrong time or can’t close tightly enough, acidic stomach contents wash upward. Roughly 18 to 28 percent of North American adults experience this regularly enough for it to be classified as gastroesophageal reflux disease, or GERD.
The Valve That Keeps Acid Down
At the junction where your esophagus meets your stomach sits a band of muscle called the lower esophageal sphincter, or LES. This sphincter stays contracted at rest, creating a seal that keeps food and acid from traveling upward. When you swallow, nerve signals trigger a brief relaxation so food can pass through into the stomach, then the muscle tightens again.
The problem in acid reflux is that the LES relaxes when it shouldn’t, or it doesn’t close with enough force. The sphincter is made up of both esophageal muscle fibers and fibers from the diaphragm, the breathing muscle that sits just above your stomach. Both sets of muscles need to work together to form a competent seal. When either component weakens or the timing of relaxation goes wrong, the barrier fails and stomach contents escape upward.
What Stomach Acid Actually Does to Your Esophagus
Your stomach produces hydrochloric acid with a pH around 2, which is strong enough to break down food. The stomach has a thick mucus lining that protects it from this acid. The esophagus does not.
When acid reaches the esophagus, the real damage involves more than just a chemical burn. Stomach juice contains an enzyme called pepsin, which is designed to digest proteins. Pepsin is most active at a pH of 2.0, but here’s the key detail: it doesn’t die when conditions become less acidic. It simply goes dormant and remains stable at a pH as high as 8.0. That means pepsin can coat the esophageal lining, sit there quietly, and reactivate the next time acid washes up.
Once reactivated, pepsin breaks down the proteins that hold esophageal cells together. It degrades the “glue” between cells and widens the gaps in the tissue, making the lining more permeable. This lets acid seep deeper into the tissue layers. The process also generates oxidative stress, damaging the energy-producing structures inside cells and triggering inflammatory responses that compound the injury over time. This is why chronic reflux causes progressive damage rather than just momentary discomfort.
Why Certain Foods Make It Worse
Some foods directly weaken the LES. Chocolate contains a compound called methylxanthine, which is chemically similar to caffeine. Both relax the sphincter muscle. Coffee does this whether it’s caffeinated or decaf. Fatty, spicy, and fried foods have a double effect: they relax the LES and slow stomach emptying, which means more acid sits in the stomach for longer, increasing the odds it will reflux upward.
Large meals create additional pressure inside the stomach, physically pushing contents toward the weakened valve. Eating close to bedtime compounds the problem because lying down removes gravity’s help in keeping acid down.
How a Hiatal Hernia Changes Things
In some people, the upper portion of the stomach pushes up through the opening in the diaphragm where the esophagus passes through. This is a hiatal hernia, and it directly undermines the anti-reflux barrier. When the junction between the esophagus and stomach rises above the diaphragm, the diaphragm muscles can no longer squeeze the sphincter shut effectively. The hernia also traps a pocket of acid at the top of the stomach that can’t drain back down, creating a reservoir that washes into the esophagus easily.
Symptoms Beyond Heartburn
The classic symptom is a burning sensation behind the breastbone, but acid reflux can cause problems far from the esophagus. When small amounts of stomach contents reach the throat, voice box, or airways, they trigger a separate set of symptoms: chronic cough, hoarseness, a feeling of a lump in the throat, and worsening asthma.
This happens through two routes. The first is direct contact. When refluxed material reaches the larynx or lungs, pepsin and acid irritate those tissues directly. Pepsin can enter the cells of the throat lining and damage them even in non-acidic conditions, which is why some people on acid-suppressing medications still have throat symptoms. The second route is indirect. Acid in the esophagus stimulates nerve pathways that trigger reflexive coughing and airway tightening, even when nothing actually reaches the lungs. Research has shown that esophageal acid exposure can cause the release of inflammatory substances in lung tissue through this nerve-mediated pathway.
How Reflux Medications Work
The two main categories of acid reflux medication target acid production at different points. H2 blockers (like famotidine) block one of the chemical signals that tells stomach cells to produce acid. They start working within one to three hours. Proton pump inhibitors, or PPIs (like omeprazole), go a step further by shutting down the actual acid-producing pump inside stomach cells. PPIs take longer to kick in but suppress acid more completely and for a longer duration.
Neither type of medication fixes the faulty valve. They reduce the acidity of what refluxes, which limits tissue damage and eases symptoms. This distinction matters because pepsin can still reach the esophagus and throat on non-acidic reflux, which partly explains why some people continue to have symptoms despite medication.
Sleep Position and Nighttime Reflux
Nighttime reflux tends to cause more damage because you swallow less frequently during sleep, so acid sits in the esophagus longer. Elevating your upper body with a wedge pillow helps by using gravity to keep acid in the stomach. Research from Harvard Health found that sleeping on your left side also makes a measurable difference. It didn’t reduce the number of reflux episodes, but acid cleared from the esophagus significantly faster compared to sleeping on the back or right side. The anatomy explains this: when you lie on your left, the stomach hangs below the esophageal junction, making it harder for acid to pool at the opening.
Long-Term Risks of Untreated Reflux
Chronic acid exposure can change the cells lining the lower esophagus. In a condition called Barrett’s esophagus, the normal flat cells are replaced by taller, column-shaped cells that resemble intestinal lining. This is the body’s attempt to protect itself from ongoing acid injury, but the new cells carry a small risk of becoming cancerous. In people with Barrett’s who show no precancerous changes, the annual risk of developing esophageal cancer is 0.12 to 0.40 percent. That risk climbs to about 1 percent per year when early precancerous changes are present, and above 5 percent per year with more advanced changes.
These numbers are low in absolute terms, but they accumulate over years and decades, which is why people with long-standing, frequent reflux are typically monitored with periodic endoscopy to catch changes early.
How Reflux Is Confirmed
Most people are diagnosed based on their symptoms and their response to a trial of acid-suppressing medication. When the diagnosis is uncertain, a pH monitoring test measures acid exposure directly. A thin probe placed in the esophagus records pH levels over 24 to 48 hours. The key measurement is the percentage of time the esophageal pH drops below 4.0. If that percentage exceeds 4.3 percent of the total recording time, acid exposure is considered abnormal. For someone already taking acid-suppressing medication during the test, the threshold drops to 1.3 percent.