Heartburn 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 stays closed to keep acid where it belongs, but when that muscle relaxes at the wrong time or becomes weakened, acid escapes upward and irritates the esophageal lining. This produces the familiar burning sensation behind your breastbone. An estimated 10 to 20 percent of adults in Western countries experience it regularly, and the causes range from what you ate for dinner to underlying anatomical changes in your body.
The Muscle That Keeps Acid in Place
The lower esophageal sphincter, or LES, is a high-pressure zone where the esophagus meets the stomach. Think of it as a one-way valve: it opens to let food drop into the stomach, then squeezes shut to prevent anything from coming back up. When this valve works properly, you never feel stomach acid in your chest.
The most common cause of heartburn is something called a transient relaxation of this sphincter. The valve opens briefly when it shouldn’t, allowing acid to splash upward. These inappropriate relaxations happen in everyone occasionally, but they occur more frequently in people with chronic reflux. The sphincter can also become chronically weak due to certain foods, medications, smoking, or structural changes in the body, all of which reduce the pressure it generates.
Foods and Drinks That Trigger Reflux
Certain foods directly reduce the pressure your esophageal sphincter can maintain. High-fat meals, chocolate, alcohol, and carbonated beverages all lower sphincter pressure and increase acid exposure in the esophagus. Peppermint and other mints have a similar relaxing effect on the muscle. These aren’t just irritating your stomach; they’re physically loosening the valve that protects your esophagus.
Other foods trigger heartburn through a different route: they irritate the esophageal lining on contact. Spicy foods, citrus fruits, tomatoes, onions, and garlic fall into this category. Caffeinated beverages can contribute through both mechanisms, relaxing the sphincter while also stimulating acid production. If you notice heartburn after specific meals, the culprit is likely one of these common triggers.
Eating large meals also matters, regardless of what’s on the plate. A full stomach increases pressure against the sphincter, making it more likely that acid will push through. Eating within two to three hours of lying down compounds the problem because gravity is no longer helping keep acid in your stomach.
How Body Weight Affects Heartburn
Carrying extra weight, especially around your midsection, is one of the strongest risk factors for heartburn. Abdominal fat increases the pressure inside your abdominal cavity by pressing down on your stomach, which in turn pushes acid upward toward the esophagus. The numbers are striking: in one study, heartburn symptoms were present in about 23 percent of people with a normal BMI, 27 percent of those who were overweight, and 50 percent of those with obesity.
This isn’t just about overall weight. Abdominal obesity specifically transmits gravitational force from fat tissue into the abdominal cavity, creating persistent upward pressure on the stomach. Even modest weight loss around the midsection can meaningfully reduce how often heartburn occurs.
Hiatal Hernia and Structural Changes
A hiatal hernia occurs when the upper part of your stomach pushes up through the opening in your diaphragm, the large muscle separating your chest from your abdomen. Normally, your diaphragm reinforces the esophageal sphincter from the outside, like a second layer of defense. When a hiatal hernia develops, these two structures separate, and neither works as well on its own.
The consequences are significant. A hiatal hernia reduces the length and pressure of the sphincter, impairs the reinforcing squeeze of the diaphragm, decreases the wave-like contractions that clear acid from the esophagus, and creates a small reservoir of acid in the herniated portion of the stomach that can reflux upward during swallowing. Many people with small hiatal hernias have no symptoms at all, but larger hernias can make heartburn both frequent and difficult to control.
Medications That Cause or Worsen Heartburn
Two distinct groups of medications contribute to heartburn, and they do it in different ways. The first group directly irritates the esophageal lining as the pills travel down to your stomach. Common painkillers like ibuprofen and aspirin fall into this category, along with certain antibiotics, iron supplements, and bone-density medications like alendronate. Taking these on an empty stomach or without enough water makes the irritation worse.
The second group loosens the esophageal sphincter itself, making reflux more likely. This includes some blood pressure medications (calcium channel blockers and nitrates), certain antidepressants, sedatives like benzodiazepines, opioid pain medications, and progesterone. If you started a new medication around the time your heartburn began, the timing may not be a coincidence.
Pregnancy and Hormonal Changes
Heartburn affects the majority of pregnant women, particularly in the second and third trimesters. Progesterone, a hormone that rises sharply during pregnancy, relaxes smooth muscle throughout the body. That includes the esophageal sphincter, which loses some of its ability to stay tightly closed. Partly digested food and stomach acid can then flow backward into the esophagus more easily.
As the pregnancy progresses, the growing uterus also pushes upward on the stomach, adding physical pressure to the hormonal effect. This combination explains why heartburn tends to worsen as the due date approaches. It also explains why heartburn typically resolves soon after delivery, once progesterone levels drop and the physical pressure on the stomach is gone.
Smoking and Nicotine
Cigarette smoking reduces esophageal sphincter pressure by 19 to 42 percent, a substantial weakening of the valve that protects your esophagus. Nicotine itself is the primary culprit. Studies using nicotine patches (removing the effects of smoke inhalation entirely) found that nicotine alone reduced sphincter pressure by about 27 to 31 percent in healthy subjects. This means nicotine replacement products, while far safer than cigarettes for your lungs, can still contribute to heartburn.
Smoking also reduces saliva production. Saliva is mildly alkaline and helps neutralize any acid that reaches the esophagus, so producing less of it means acid lingers longer and causes more damage.
Why Heartburn Gets Worse at Night
Lying flat removes gravity from the equation. When you’re upright, gravity helps keep stomach contents where they belong. When you lie down, acid can flow sideways into the esophagus much more easily, and your body clears it more slowly.
Your sleeping position matters more than you might expect. Lying on your right side positions the esophagus below the junction where it meets the stomach, essentially pouring acid toward the valve. Sleeping on your left side does the opposite: it places the esophagus above the stomach, so acid has to travel uphill to reach it. Elevating the head of your bed by six inches (using blocks under the bedframe, not just extra pillows) also helps by tilting your entire torso.
When Heartburn Signals Something Serious
Occasional heartburn after a heavy meal is normal. Persistent heartburn that occurs more than twice a week may indicate gastroesophageal reflux disease, or GERD, which can damage the esophageal lining over time. Certain symptoms alongside heartburn warrant prompt medical attention: difficulty swallowing, pain while swallowing, unexplained weight loss, signs of gastrointestinal bleeding (such as dark or bloody stools), persistent vomiting, or unexplained iron-deficiency anemia. These are considered alarm symptoms that typically lead to an endoscopy to check for complications like narrowing of the esophagus, ulceration, or precancerous changes in the tissue.