What is the Difference Between An Abscess And An Ulcer?
Written By Kobi Nathan, Pharm.D., M.Ed., CDP, BCGP, AGSF
Sleep Disorders
September 23, 2023

Skin abscesses and ulcers are both lesions. Lesions are abnormal changes observed on the skin.

They are clearly different from the surrounding skin or tissue. 

Abscesses and ulcers can occur anywhere on or inside the body. 

For example, an abscess can form at the site of your new hip or knee replacement inside your body. 

Also, you most likely have heard of stomach ulcers. 

That said, the remainder of this article will focus on abscesses and ulcers that form on the skin.

There are some key differences between these two types of skin lesions. 

Specifically, an abscess is a closed lesion with a pocket of pus under the skin, while an ulcer is an open sore of the skin. 

Additionally, abscesses tend to be round and firm, while ulcers are usually shallow and crater shaped.

Below are two images showing what an abscess and an ulcer look like:

Elbow with abscess


Appears red, angry, raised, and swollen. The skin directly over the center of the abscess can appear thin. The abscess looks white or yellow because of the pus underneath it.



Appears cratered, open sore with clear serous fluid.


Both conditions can be painful, and both can cause infection. 

An abscess can rupture and release the pus into the surrounding tissues if left untreated. 

This can cause further inflammation and pain.

Alright, let’s dive a little deeper into each type of lesion. Let’s discuss abscesses first…

What causes an abscess to form?

Most abscesses are caused by a bacterial infection when bacteria enter the skin through a break or scratch.

In certain rare cases, abscesses can also be caused by viruses, fungi, or parasites.

Once the bacteria have penetrated the body’s main form of defense against the environment, they are free to take root and multiply unopposed. 

The bacteria now have direct access to warmth, a wet environment, and plenty of nutrients such as glucose and proteins, the very ingredients they need to grow quickly and exponentially.

Now, the human body being the amazing machine that it is, immediately identifies that a foreign organism has invaded it and begins the process of walling off the offending pathogen from spreading to other parts of the body. 

Specifically, the body’s immune system sends white blood cells to the target area to fight the bacteria. 

With the fight raging, inflammation results, and the tissue in the immediate vicinity and surrounding area are destroyed. 

As a result, a cavity is created filling with pus, and thus, an abscess is formed.

The pus is filled with white blood cells, dead bacteria, and dead tissue.  

While this process prevents the pathogen’s spread and keeps it localized, there is a downside – antibiotics cannot penetrate the abscess to start killing the bacteria.  

The other problem is that the abscess can get larger and more painful as more white blood cells are sent to the site of infection as the bacteria put up a good fight.

What type of bacteria causes abscesses to form?

The main culprit is the gram-positive bacteria, Staphylococcus aureus (commonly called “staph infection”).  

There are many different types (strains) of Staphylococcus bacteria. 

Staph infections are extremely common in the United States. 

Millions of staph infections are diagnosed every year. 

While most are mild, some cases can result in serious complications.

You are at the highest risk of getting abscesses if you:

  • Inject drugs
  • Are in the hospital, have medical devices, hardware, or catheters in your body
  • Have had recent surgery
  • Have a chronic medical problem such as diabetes (higher risk for infections), eczema (due to constant scratching and breaking of skin), or disease affecting your blood vessels
  • Have a weakened immune system (either due to disease or medications that lower your immune response)
  • Have congenital heart defects, such as heart valve defects
  • Wear a tampon for longer than recommended
  • Have had any type of surgery on your heart valves


Skin abscesses in the elderly

Skin infections and abscesses are common in the elderly. This is due to the following:

  • Changes in the older adult’s skin integrity and structure
  • Immunosenescence (gradual decline in the elderly’s immune function due to the natural aging process)
  • Chronic medical condition such as diabetes, eczema, etc


Dangers of skin abscesses in the elderly

Quickly diagnosing and treating skin infections is very important in the elderly.

Untreated infections can quickly lead to a decline in functional status, frailty, hospitalization, and premature death.


How is an abscess treated?

Most small abscesses can resolve on their own. 

Make sure you are not picking at them or trying to break the abscesses to drain them yourself. 

You may inadvertently introduce more bacteria to the infection site or break the surrounding healthy tissue, causing further infection to spread.

Your healthcare provider may decide to do a surgical debridement or incision and drainage (called an “I & D” in medical speak) by cleaning the area on and around the abscess, then making an incision in the abscess to drain the pus and fluid. 

Depending on the abscess size, your doctor may pack the space inside the abscess with gauze. 

The incision is left open, covered with a clean bandage, and left to heal and close up on its own. 

Your doctor may also decide to prescribe antibiotic treatment to make sure your abscess site heals completely and does not get re-infected.

What causes ulcers?

As I noted earlier, ulcers are sores that can occur either on the skin or inside the body. 

They tend to be painful and penetrate much deeper into the skin and tissue than abscesses do. 

Inside the body, they can occur along the stomach lining (peptic ulcer), small intestine (duodenal ulcer), or esophagus (esophageal ulcer).

These are the GI ulcers that most often come to mind when we think of ulcers.

Regarding these GI ulcers, the likely causes are certain medications (eg., NSAIDs), stress, bacteria, as well as damage inflicted on sensitive, vulnerable tissue by existing body fluids or chemicals.

Let me explain in more detail…

A Helicobacter Pylori (H. pylori) infection in the stomach is the main bacterial cause of gastritis and resulting gastric ulcers. 

Between 30%-40% of people in the United States are estimated to be infected with H. pylori.

On the other hand, gastroesophageal reflux disease, more commonly called GERD, causes stomach acid to rise up into the esophagus, irritating and damaging the delicate and sensitive tissue.

This can also lead to ulceration of the esophagus.

If left untreated, gastritis and GERD can lead to long-term, irreversible damage and possibly cancer. 

You can read more about these two medical problems and how to manage them in my article here.


Pressure or Bed sores

pressure sore locations on body

Since we are talking about skin ulcers and our focus on this website focuses on the elderly, let’s delve briefly into a type of ulcer very commonly seen in frail, bedridden older patients – pressure ulcers (also called decubitus ulcers, bedsores, or pressure sores). 

The most common areas that are at risk for getting pressure sores are any bony protruberances of the body that make direct, constant contact with the environment. 

Specifically, the heels, elbows, back of the shoulders, back of the head, sacrum (base of the spine, also commonly called the “tail bone”), and hips.

Older adults are particularly at risk for getting pressures ulcers because they tend to sit for extended periods of time in their wheelchairs (pressure on the tailbone) or are bedridden (pressure on the hips and elbows)

The prolonged pressure on the fragile skin causes injury to the skin tissue in the affected area, resulting in tissue breakdown.

Pressure ulcers tend to develop slowly, but some can form in a matter of hours! In severe cases, these can lead to sepsis, a blood infection, and death.

The table below describes early and late symptoms of pressure ulcers with a stage-wise description of tissue destruction:

Signs and Symptoms of Pressure Ulcers

Early SymptomsLater Symptoms
Skin becomes discolored (red, blue, or purple patches)Can form open wound or blister (Stage 2 ulcer)
Discolored patches don't turn white when pressedDeep wound reaches deeper layers of skin (Stage 3 ulcer)
Skin feels warm, spongy or hardVery deep wound reaching muscle and bone (Stage 4 ulcer)
Pain or itchiness in affected skin


There are many other types of ulcers:

  • Corneal ulcers – open sore on the cornea caused by a bacterial, parasitic, viral, or fungal infection
  • Mouth ulcers (mouth sores):
    • Canker sores (aka aphthous ulcers) – caused by food allergies, stress, vitamin deficiencies, infections, hormone changes, painful sores on the mucous membrane
    • Cold sores – caused by Herpes Simplex virus type 1 (oral herpes), contagious, painful sores
  • Diabetic foot ulcers – caused by uncontrolled diabetes, resulting in poor blood flow, peripheral neuropathy (nerve damage)
  • Arterial ulcers – caused by diabetes, high blood pressure, Arteriosclerosis (hardening of the arteries)
  • Leg ulcers or venous skin ulcers (caused by vascular insufficiency or poor blood circulation)


How to treat an Ulcer

It depends. Stomach ulcers caused by an H. pylori infection are treated with a very intensive regimen of antibiotic therapy and proton pump inhibitors (PPIs) which are used to reduce the irritation on the stomach lining, allowing it to heal.

Esophageal ulcers caused by acid reflux and GERD are treated primarily with PPIs or H2 receptor antagonists (Pepcid, Tagamet, etc).

Pressure ulcers are treated with constant repositioning of the bedridden or wheelchair-bound older adult to shift weight and pressure away from the bony prominences of the body, and cleaning, dressing, and packing of existing ulcers.   


  • Abscesses are closed, pus-filled lesions, while ulcers are open sores
  • The most common cause of abscesses is bacteria, specifically Staphylococcus Aureus
  • Abscesses are treated by incision and drainage, and antibiotics
  • Ulcers can be caused by Helicobacter pylori bacteria, stress, hormone changes, certain medications, or GERD
  • Ulcers are treated with PPIs, lifestyle management, and pressure-relieving body repositioning, depending on the type of ulcer
  • Both ulcers and abscesses must be treated promptly to minimize worsening of quality of life, irreversible damage, and hospitalization and/or premature death


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