Veins are blood vessels that carry blood from all the organs toward the heart. The blood in the veins is dark red (it may appear to be purple, dark blue or even black). This is the normal color of venous blood, because the arterial blood filled with oxygen (oxygenated blood) passes oxygen on to the cells and goes to the veins, where it darkens (deoxygenated blood).

Thin mucous membranes, such as that of the bottom of the tongue, can be transparent (almost translucent), so there we can easily spot the veins and their darker blood. This is most often not a sign of any serious health problems.

Certain conditions may cause caviar tongue (sublingual varicosities), where the color of the veins may vary (blue, dark, black, purple). These are:

• Mechanical damage to the tongue – occurs as a result of chewing, injuries caused by wearing braces or dental procedures.
• Certain medications, such as aspirin or blood clotting medications can cause bruising and caviar tongue. Because of the thin mucous membrane, the veins under the tongue are important for drug absorption, so a lot of medications are absorbed under the tongue.
• Varicose veins usually occur in the elderly. These veins can most often be seen on legs, but also under the tongue. Varicose veins may vary in size and intensity, but they usually have no consequences and do not require treatment.
• Long lasting dental implants in the mouth, such as braces, can cause an allergic reaction that may be visible throughout the entire oral cavity, including the tongue – these changes appear as redness and swelling.
• Hemangioma – a benign tumor that forms as an abnormal collection of blood vessels. It is not dangerous and malignant, it does not metastasize, and can be located on the lower surface of the tongue. Since it has the structure of a blood vessel, our eye perceives it as sublingual varicosities.
  
• Some other conditions such as: inflammation of salivary glands, vitamin B12 deficiency, smoking, weak immune system, infections (bacterial and viral), and hormone changes in the body.