An open sore on the outside of your body is painful and gets easily irritated, but it's visible: You can see it and apply a topical medication or a bandage. Things are more complicated when the sore is inside -- also known as an ulcer. There are several different kinds, based on where they form, but none is pleasant and most can strike just about anybody. And it’s important to catch them before they worsen and cause internal bleeding.

Esophagus

Although ulcers in the esophagus — the tube connecting the mouth and the stomach — are rare, several common conditions can cause them. Acid reflux is the most common, according to the Journal of Gastrointestinal Surgery, on average creating an ulcer that is larger than 2.5 centimeters. That’s significant because between 14 and 20 percent of American adults have gastroesophageal reflux disease, the condition in which stomach acid backs up and irritates the esophagus’ lining, and which is marked by frequent heartburn. The ulcer opens up when the issue is left untreated and acid erodes the lining.

Read: Signs of Serious Heartburn

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But people also get the ulcers from certain drugs, smoking cigarettes, or an infection such as a fungus, herpes or HIV. Bulimia, the eating disorder in which people purge what they’ve eaten, can be a culprit as well, as the Kansas City-based Saint Luke’s Health System notes that excessive vomiting can lead to an ulcer in the esophagus.

People with an esophageal ulcer might have trouble and pain swallowing, heartburn, nausea and chest pain, and could vomit blood.

Stomach and small intestine

These two types of ulcers are both part of the category called peptic ulcers. The Mayo Clinic explains that when they show symptoms at all, they are most often accompanied by burning stomach pain that will get worse on an empty stomach. But they can also cause symptoms like bloating, belching, heartburn and nausea or, in more extreme cases, vomiting blood, blood in the stool, trouble breathing, changes in appetite and weight loss. Those latter two symptoms would occur because the sore is responsible for swelling or scarring along the digestive tract.

According to the Mayo Clinic, an infection and long-term use of certain over-the-counter painkillers are the most common causes. Those painkillers could be aspirin, ibuprofen medications like Advil and Motrin, or naproxen sodium pills such as Aleve.

When these peptic ulcers are in the stomach, they are called gastric ulcers, while the ones in the upper part of the small intestine are duodenal ulcers. Both stress and spicy foods could make them worse.

Read: 7 Places You Can Bleed from and What It Means

Stress

Our old friend, stress, is at it again. However, the internal sores are linked to physical stress — this type of ulcer is superficial and only occurs under the most extreme circumstances. Patients who are critically ill, like those in an intensive care unit, are at risk, according to a report in Seminars in Gastrointestinal Disease, particularly those who are on a ventilator to support or completely control their breathing or patients whose blood is not clotting properly. For these people, ulcers are a bad sign: “The presence of gastrointestinal bleeding in a critically ill patient predicts markedly increased mortality.”

Rectum

Because of its very nature, people may feel uncomfortable talking about a sore on their rectum, the tube for stool that leads from the end of the large intestine to the anus. These sores are often found in people with chronic constipation, the Mayo Clinic says, and can cause bleeding and difficulty pooping, as well as incontinence, mucus, pain, a feeling of fullness in the pelvis or one of incompleteness after a bowel movement. In some people, there are no symptoms.

In addition to constipation, the Mayo Clinic lists various injuries that could cause rectal ulcers, including rectal prolapse, in which the rectum protrudes from the anus; and impacted stool or an attempt to manually remove it.

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