A blood blister is a fluid-filled sac that forms under the top layer of skin. It can also form under a moist layer of tissue inside the mouth or other body cavity. Blood blisters are common and can occur when your skin is rubbed or pinched, such as by a tight-fitting shoe or catching your finger in a door.

Typical blood blisters are usually harmless and self-treatable, healing within 1–2 weeks. However, if bacteria enter the blister and infect it or the blister does not heal by itself, contact a medical professional to ensure it is not a more serious condition. 

In this article, you will learn about blood blister symptoms and causes. You will also get first-aid tips for a blood blister and find out when to see a doctor for professional treatment.

A blood blister starts underneath the top layer of the skin and forms a fluid-filled sac. Blood leaks into the sac from damage to small blood vessels. Blood blisters arise from high-pressure rubbing or pinching, which tends to cause a deeper injury compared with friction that leads to regular blisters.

Blood blisters initially appear red or dark purple, and they can later turn black as they heal and the blood dries. 

Blisters may be painful or itchy. They can also be flat or have a raised, bubble-like sac. The size of these blisters ranges from tiny up to 1 inch or more in diameter. You can have more than one blood blister. Most blood blisters heal by themselves.

Blood blisters usually occur on the feet, toes, hands, and fingers. However, they can also appear in other locations on the body.

High-pressure rubbing, chafing, or pinching can lead to blood blisters. Some additional blood blister causes include:

If a dark spot appears without prior injury, you may want to consult with a dermatologist to rule out melanoma, a type of skin cancer. 

What does melanoma look like? Find out here.

Blood blisters in the mouth — inside your cheek or on your tongue — are typically harmless and often heal on their own. Cheek-biting or accidentally biting your tongue can cause them. However, they can also develop from food or drug allergies, very hot food, vitamin C or B12 deficiency, and herpes.

Blood blisters in the mouth may also be related to an uncommon disease called angina bullosa hemorrhagica (ABH). This occurs mostly in older adults and causes blood blisters frequently on the roof of the mouth. People with diabetes may have a higher likelihood of developing ABH. However, the blisters are harmless and usually heal quickly on their own. 

You may consider contacting your doctor if the blister does not improve in a few days or it shows signs of infection, including:

If blood blisters appear suddenly with no injury or in the eyelids, mouth, or genitals, contact a medical professional promptly. They can review your medical history and symptoms. Depending on your situation, they may also order blood tests and take a tissue sample, or biopsy, of the blister for testing.

If you have a blood blister without signs of infection, you can treat it at home. The most important thing is to stop the activity that caused the blister. Here are some home treatment recommendations for blisters:

Your doctor may drain a blood blister if there is an infection or if it is very large and painful. They may prescribe antibiotics to help it heal.

Here are answers to other questions people often ask about blood blisters.

No, it is best to let the blister heal on its own, especially if you have diabetes or a compromised immune system. These conditions can make it more difficult for the body to fight off infection, and popping the blister increases the chance of infection. 

Contact a clinician if the blister worsens or does not heal.

Yes, this may cause additional pain and discomfort. If it bursts, keep it clean and cover it with a bandage. 

Yes, this is part of the normal healing process. When the blood begins to dry, it will turn from red to purple. 

Do not pop a blood blister or peel the skin off as it heals. Your skin is a protective barrier against infection. Contact your doctor immediately if you do notice signs of infection. Your doctor can evaluate the severity of the infection and develop a treatment plan if necessary.

A blood blister is a collection of blood and fluid under the skin or mucous membrane. Pinches and rubbing are common causes.

Blood blisters usually heal with home treatment. However, if you notice signs of infection, increasing pain, or a blood blister appearing without any related injury, a professional medical evaluation may be wise.

You can help prevent blisters and blood blisters on your feet by wearing proper-fitting shoes and moisture-wicking socks. Wear gloves to protect your hands when using tools. If you feel pain, stop the activity if possible.

Do not attempt to pop a blood blister since this can lead to infection. 

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