A blood blister can be alarming at first glance—a deep red or purplish bubble under the skin that seems to show up without warning. While blood blisters are usually harmless, they can be painful, inconvenient, and in some situations may hint at an underlying concern. Understanding what they are and how to care for them helps you handle them correctly and recognize when medical attention may be appropriate.
What Is a Blood Blister?
A blood blister forms when tiny blood vessels (capillaries) beneath the skin rupture, while the outer layer of skin remains intact. The blood that leaks out becomes trapped under the surface, creating a raised, fluid-filled spot that appears red, maroon, or purple—often much darker than a typical clear blister.
Blood blisters most often show up on:
- Fingers and palms
- Toes and soles of the feet
- Inside the mouth (cheeks, tongue, or gums)
Common Causes of Blood Blisters
Most blood blisters are caused by sudden pinching or repeated friction, such as:
- Slamming a finger in a door
- Wearing tight or poorly fitting shoes
- Using tools or sports equipment without protection
- Repetitive movements that create ongoing pressure on the skin
Blood blisters inside the mouth can be triggered by:
- Accidentally biting the cheek or tongue
- Dental procedures
- Sharp or poorly fitting dental appliances
In some cases, blood blisters may form more easily in people with:
