A blue toenail, or subungual discoloration, is a noticeable change in color beneath the nail plate, ranging from dark purplish-blue to slate-gray. This symptom occurs when the tissue under the nail (the nail bed) reflects a change in blood supply or the presence of an abnormal substance. While often the result of a common, minor injury, this discoloration can also signal a more serious underlying issue related to circulation, oxygen levels, or systemic health.
Discoloration Due to Physical Injury
The most frequent cause of a blue or black toenail is a subungual hematoma, which is simply a bruise beneath the nail plate. This condition develops when trauma causes the tiny blood vessels in the nail bed to rupture and bleed. Since the nail plate seals the area, the blood has nowhere to go, leading to a collection of dark, clotted blood that appears blue or purple through the nail.
Direct, acute injuries, such as stubbing a toe or dropping a heavy object, are classic triggers for a subungual hematoma. However, the condition can also arise from repetitive, micro-trauma, often seen in athletes whose toes repeatedly strike the front of a shoe during activities like running or hiking. Ill-fitting footwear, whether too tight or too loose, commonly generates this type of stress, causing minor but continuous damage to the nail bed.
The discoloration initially appears reddish-purple but quickly darkens to blue-black as the blood clots under the nail. Significant blood collection can cause intense, throbbing pain, which is relieved by a medical procedure that drains the trapped blood. For small, less painful hematomas, self-care involves rest, elevation, and applying ice.
Healing is a slow process because the discolored area does not fade like a bruise; it must grow out with the nail. Since toenails grow slowly (approximately 1 millimeter per month), the blue discoloration can persist for a year or more until the nail plate is replaced. Monitoring the injury is important because hematoma pressure can cause the nail to loosen and potentially fall off.
Causes Related to Blood Flow and Oxygen Levels
When a blue tinge affects multiple nails, or appears without any history of injury, it may indicate a systemic issue related to circulation or oxygen saturation, a condition known as cyanosis. Cyanosis occurs because oxygen-depleted blood, which is darker, appears blue or purple through the nail bed. It is classified as peripheral cyanosis (a problem with local blood flow to the limbs) or central cyanosis (low oxygen levels in the entire bloodstream).
Peripheral cyanosis is often caused by conditions that restrict blood flow to the toes, such as exposure to severe cold, which causes blood vessels to narrow temporarily. A more chronic cause is Raynaud’s phenomenon, a disorder where the small arteries in the toes and fingers spasm in response to cold or stress. During a Raynaud’s episode, the affected area typically turns white, then blue, and feels cold and numb due to the temporary lack of blood flow.
Peripheral Artery Disease (PAD) is another vascular cause, where plaque buildup narrows the arteries supplying blood to the legs and feet. Unlike Raynaud’s vessel spasm, PAD is a structural issue leading to consistently reduced blood flow, often causing chronic pain, numbness, and discoloration in the extremities.
Central cyanosis points to problems with the heart or lungs, which are responsible for oxygenating the blood. Conditions like congenital heart defects, heart failure, or severe respiratory diseases prevent the blood from carrying enough oxygen. In these cases, the blue discoloration is not limited to the toenails but is also observed in the lips and skin, often accompanied by symptoms like shortness of breath or dizziness.
External Agents and Systemic Medications
Less commonly, a blue toenail can result from external contact or as a side effect of systemic treatment, rather than trauma or circulatory disease. Certain compounds can deposit in the nail bed, creating a blue-gray or slate discoloration. Exposure to heavy metals, such as silver, can cause a permanent blue-gray discoloration of the skin and nails known as argyria.
Oral medications can trigger this change through drug-induced hyperpigmentation. The antibiotic Minocycline, for instance, is known for causing blue-black or slate-gray pigmentation in the skin and nails, usually after prolonged use. This discoloration occurs when the drug’s metabolites deposit in the tissues, including the nail matrix and nail bed.
Other external factors are more benign, such as dyes leaching from footwear or hosiery. Dark socks or shoe linings can transfer color to the nail plate, especially when the feet sweat. Unlike internal causes, this discoloration is superficial and will fade quickly once the source is removed or can often be scraped away.
Identifying When to Consult a Doctor
While many blue toenails are benign and resolve with time, consulting a healthcare provider is warranted if certain signs are present. Immediate medical attention is necessary if the discoloration is associated with severe pain, especially if it appears suddenly without injury. This sudden onset without trauma could indicate a serious vascular event or an infection.
Seek professional care if there are signs of infection, such as increasing redness, warmth, swelling, or pus around the nail. If the blue color affects several nails and is accompanied by symptoms like shortness of breath, chest pain, or dizziness, it may signal central cyanosis and a serious heart or lung condition.
See a doctor if the discoloration is spreading, persistent, or fails to grow out with the nail over several months. A blue spot not clearly linked to trauma or appearing as a thin, dark line should be evaluated to rule out rare conditions, including a mole or, rarely, a form of skin cancer beneath the nail. A medical professional can accurately diagnose the cause and determine if the symptom is cosmetic or a sign of an underlying health issue.