Four common causes of blisters

A blister is a small, fluid-filled bubble on the skin that can happen for several reasons, including friction, heat, injury, or an underlying medical condition. Here are four of the most common causes of blisters.

Friction blisters

A friction blister is a common skin blister that happens when frequent rubbing or pressure against the skin causes the outermost layer of skin to separate from the other layers. Then the gap between these layers of skin fills with fluid or, if the friction is significant enough, with blood.  

A few factors that can increase the risk of friction blisters include: 

  • New socks or shoes that may rub over the skin
  • Physical occupation that requires repetitive motion
  • Poorly fitting clothes or shoes that rub against your skin 
  • Socks that are too thin or don’t remove moisture from the feet
  • Sweating excessively due to exercise or a humid environment

The most common location for friction blisters is on the feet since tight or ill-fitting shoes worn for extended periods can irritate the skin on the heels and toes.  To prevent infection, it is best not to “pop” friction blisters.

Heat blisters

Blisters can be caused by exposure to heat or sunburn. Typically a first-degree burn is a superficial heat injury that causes the skin to turn red and appear like a sunburn. A first-degree burn does not cause skin blisters.

A second-degree burn is a partial-thickness heat injury that causes the skin to turn red and blister. A third-degree burn is a full-thickness skin injury that causes the skin to turn white or black. 

Second-degree burns are one of the most common causes of skin blistering. A second-degree burn can be caused by exposure to something hot, severe sunburn, or frostbite. 

While most heat blisters are caused by accidentally touching something hot,  regularly using sunscreen can help prevent blisters from sun exposure. Frostbite can be prevented by wearing proper clothing or avoiding exposure to extreme temperatures during the winter.

It’s important to note that if you sustain a burn that creates a blister, it’s best not to “pop” the blister because it can create an opening for bacteria to enter the skin and cause an infection.

Blood blisters

A blood blister is similar to a friction blister; instead of being filled with clear fluid, it is filled with blood. It indicates that there has been damage to the blood vessels under the skin, causing them to bleed.

Due to the blood inside, the blister may look red, purple, or black. Typically blood blisters occur due to an acute injury like pinching your finger. Most blood blisters will heal on their own and do not require medical treatment. Like other blisters, avoid popping them to reduce the risk of infection. 

Blisters caused by medical conditions

Blisters are not just caused by heat or injuries. Underlying medical conditions can also cause them. Here are some common conditions that may cause blisters. 


Shingles are a viral infection caused by the varicella-zoster virus (VZV).   The virus usually starts as a burning sensation along a single nerve pathway called a “dermatome.” Then small red bumps appear, which evolve into a line of small, painful blisters called “vesicles.” 

It’s important to seek prompt medical care if you develop these blisters since antiviral medications that treat shingles are most effective if started within the first 48 hours of symptoms.


The herpes simplex virus (HSV) is categorized into HSV 1 and HSV 2.  The virus causes clusters of small blisters called “vesicles” to form on the mouth, known as oral herpes or “cold sores.”  It can also cause vesicles to appear on the genitals, known as genital herpes.  

HSV 1 was responsible for more than 80% of cold sores, and HSV 2 was responsible for more than 80% of genital herpes infections. However, largely due to oral sex, that distinction no longer exists, which means cold sores can be caused by HSV 2, and genital herpes can be caused by HSV 1. 

If you develop blisters around your lips or genitals, a healthcare provider can take a sample of the fluid in the blister and do a viral culture. The viral culture can accurately determine if HSV causes the blisters and whether the infection is HSV 1 or HSV 2.  Antiviral medications can also cause HSV outbreaks to resolve sooner.


Chickenpox is a viral infection caused by the varicella-zoster virus (VZV), the same virus that causes shingles. In addition to fever, headaches and fatigue, varicella causes a diffuse outbreak of small, itchy blisters. While the varicella vaccine has caused a dramatic decrease in varicella infections, people who have not received the varicella vaccine or have weakened immune systems can still contract the virus. 

If you or someone you know develops symptoms of chickenpox, it’s important to be isolated from other people since the varicella virus is airborne, and, therefore, highly contagious.

Contact dermatitis

Contact dermatitis is a type of rash caused by contact with a substance that leads to an allergic reaction to the skin. This reaction typically leads to redness and itchiness. If the reaction is severe enough, this may cause blisters to appear in the affected area.  A common cause of this is nickel skin sensitivity due to jewelry or metal alloys that are worn against the skin. 

Other common substances that may cause this reaction include plants, fragrances, personal care products, laundry detergents, or cosmetics. Avoidance of the substance is the best treatment, but corticosteroids or other oral medications can also be needed.

Hand, foot and mouth disease

Hand, foot and mouth disease is a contagious viral condition caused by coxackievirus that typically infects children under 5, although anyone can get it. Symptoms include fever, a skin rash that looks like blisters on the feet and hands, and mouth sores. This condition is not life-threatening and tends to clear up within ten days. 

Hand, foot and mouth does not usually need treatment but could need additional medical addition in certain situations. If a fever lasts more than three days or you have difficulty swallowing, you may want to see a healthcare provider. 

Treatment for blisters at urgent care

For treatment of infected skin blisters or related medical conditions, visit our urgent care centers—no appointment necessary, or reserve your spot online. Common during summer hikes and activities, friction blisters can interrupt your vacation. We offer treatments for blisters and most non-urgent health issues, ensuring you get the care you need every day of the year.



Written by Sarah Thebarge, Physician Assistant