Acid reflux happens when stomach acid flows backward into your esophagus, the tube connecting your mouth to your stomach. At the bottom of that tube sits a ring of muscle that normally opens to let food down and then closes tight. When that muscle fails, acid escapes upward. Roughly 825 million people worldwide deal with chronic reflux, making it one of the most common digestive problems on the planet.
The Muscle That Keeps Acid in Place
The lower esophageal sphincter (LES) is the gatekeeper between your esophagus and stomach. It stays contracted at rest, creating a seal that keeps acid where it belongs. Two main patterns of failure cause reflux: the muscle relaxes too often at the wrong times, or its resting pressure is simply too weak to hold the seal.
These problems stem primarily from defective nerve signaling to the muscle, though the muscle tissue itself can also weaken over time. Your diaphragm, the large breathing muscle that wraps around the base of the esophagus, normally provides backup support. When you strain, cough, or bend over, the diaphragm squeezes the esophagus shut. If either system falters, acid gets through.
How a Hiatal Hernia Disrupts the Seal
A hiatal hernia occurs when the upper part of your stomach pushes up through the opening in your diaphragm where the esophagus passes through. This pulls the junction between your esophagus and stomach above the diaphragm, separating it from the muscles that normally help squeeze the esophagus closed. With those muscles out of position, they can’t tighten enough to prevent acid from washing back up. Not everyone with a hiatal hernia gets reflux, but the larger the hernia, the more likely acid will escape.
Body Weight and Abdominal Pressure
Carrying extra weight around your midsection increases pressure inside your abdomen, which pushes against the stomach and forces acid upward. Interestingly, the relationship isn’t simply about total body weight. Research shows that it’s the distribution of fat, specifically around the belly, that matters more than overall BMI. Abdominal fat has also been linked to a higher number of inappropriate LES relaxation episodes, meaning the muscle opens when it shouldn’t, letting acid through more frequently.
Foods and Drinks That Relax the Valve
Certain foods don’t just irritate the esophagus on the way down. They actually cause the LES to loosen. Chocolate contains a compound called methylxanthine, which is chemically similar to caffeine and directly relaxes the sphincter muscle. A 2020 review of GERD research confirmed that chocolate measurably decreases pressure in the lower esophageal sphincter.
Coffee, peppermint, and high-fat meals have similar effects. Fatty foods slow stomach emptying and trigger hormonal signals that relax the LES. Acidic foods like tomatoes and citrus don’t necessarily weaken the sphincter, but they make the acid that does escape more painful because they lower the pH of your stomach contents. Alcohol both relaxes the muscle and irritates the esophageal lining, a double hit. Carbonated drinks introduce gas that distends the stomach, which can force the sphincter open.
Pregnancy and Hormonal Changes
Up to two-thirds of pregnant women experience acid reflux, and hormones are the primary reason. Progesterone, which rises dramatically during pregnancy, relaxes smooth muscle tissue throughout the body, including the esophageal sphincter. As the pregnancy progresses, the growing uterus also pushes the stomach upward, compressing it and making reflux even more likely. This combination of hormonal relaxation and physical pressure explains why symptoms tend to worsen in the third trimester and typically resolve after delivery.
Medications That Make Reflux Worse
Some medications trigger reflux in two distinct ways. One group directly irritates the esophageal lining as the pills travel down, mimicking the burning sensation of acid reflux. This includes common pain relievers like ibuprofen and aspirin, certain antibiotics, iron supplements, and bone-density medications.
A second group actually weakens the esophageal sphincter or increases acid production. Blood pressure medications (particularly calcium channel blockers and nitrates), certain antidepressants, sedatives like benzodiazepines, opioid painkillers, and overactive bladder medications all fall into this category. Progesterone supplements can have the same sphincter-relaxing effect seen in pregnancy. If you started a new medication around the time reflux symptoms appeared, the timing may not be coincidental.
Slow Stomach Emptying
Gastroparesis is a condition where the stomach takes much longer than normal to move food into the small intestine. When food sits in your stomach for hours, the stomach stretches and distends. That distension makes it physically easier for acid to escape out the top of the stomach and into the esophagus. People with gastroparesis often feel full almost immediately after eating, experience nausea, and develop chronic acid reflux as a secondary problem. Diabetes is the most common cause of gastroparesis, which is why persistent reflux in people with diabetes deserves a closer look.
Why Reflux Gets Worse at Night
Gravity is one of your best defenses against acid reflux. When you’re upright, acid that splashes into the esophagus drains back down quickly. Lying flat eliminates that advantage entirely, allowing acid to pool in the esophagus for longer periods and travel further up. Swallowing, which happens less frequently during sleep, also clears acid from the esophagus. The combination of reduced gravity, less swallowing, and reduced saliva production (which normally neutralizes acid) makes nighttime reflux more damaging to esophageal tissue. Elevating the head of your bed by about six inches, not just stacking pillows, restores some of gravity’s protective effect.
Occasional Reflux vs. Chronic GERD
Nearly everyone experiences acid reflux occasionally, especially after a large meal or a night of heavy eating. That’s normal. Gastroesophageal reflux disease (GERD) is the clinical diagnosis when reflux causes symptoms two or more times per week, or when acid exposure has already damaged the esophageal lining, regardless of symptom frequency. The distinction matters because chronic acid exposure can lead to inflammation of the esophagus, narrowing of the esophageal passage, and changes to the cells lining the lower esophagus.
The causes described above rarely act alone. Most people with persistent reflux have a combination of factors: perhaps a small hiatal hernia, some extra abdominal weight, a diet high in triggering foods, and a medication that weakens the sphincter. Identifying which factors apply to you is the first step toward reducing symptoms, because the ones you can modify (weight, diet, eating timing, body position) often make a meaningful difference even without medication.