A toe turning black almost always comes down to one of two broad categories: blood trapped under the nail from an injury, or blood that has stopped reaching the tissue properly. The first is common, usually harmless, and resolves on its own. The second can signal a vascular emergency, an infection spiraling out of control, or, less commonly, a skin cancer. Telling the difference is not always obvious at a glance, and the stakes of getting it wrong range from a lost toenail to a lost limb.
The Most Common Cause Is a Bruise Under the Nail
If you stubbed your toe, dropped something on it, or have been running in shoes that are too tight, the likeliest explanation is a subungual hematoma: blood pooling between the nail and the nail bed. The nail turns brown, dark purple, or black because you are literally looking at a bruise through a window of keratin. The discoloration can appear right away or show up hours after the injury, and it is usually accompanied by throbbing pain that gets worse with pressure.
Most subungual hematomas are harmless. The trapped blood slowly migrates forward as the nail grows out, which takes a few months on a toenail. If pressure under the nail is intense, a doctor can relieve it through a procedure called nail trephination, which involves making a small hole in the nail to let the blood drain. A prospective study of this procedure found that all patients experienced pain relief afterward, and over an average follow-up of about ten months, none developed infection or significant nail deformity. That same study concluded that for straightforward blood-under-the-nail injuries, removing the nail and suturing the nail bed is unnecessary.1The American Journal of Emergency Medicine. Treatment of subungual hematomas with nail trephination: A prospective study
There is an important caveat. If the trauma was severe enough to fracture the bone underneath, or if the nail bed is significantly disrupted, you need more than a drainage hole. And if you do not remember any injury at all, “it’s just a bruise” becomes a diagnosis you should not make on your own. A black spot under a toenail with no clear trauma history needs a closer look, because some of the causes below can mimic one.
Poor Blood Flow and the Risk of Gangrene
When arteries that supply the feet narrow or become blocked, the tissue at the tips of the toes is the first to starve. Gangrene, the death of tissue from inadequate blood supply, is a serious complication of peripheral vascular disease.2PubMed Central. Understanding Gangrene in the Context of Peripheral Vascular Disease: Prevalence, Etiology, and Considerations for Amputation-Level Determination In its “dry” form, gangrene causes the toe to turn black, shrivel, and feel hard and cool to the touch. There may be pain, or the area may be numb if the nerve supply has also been compromised.
The people most at risk are those with atherosclerosis, the gradual buildup of fatty deposits in artery walls. Smoking, high blood pressure, high cholesterol, and age all accelerate the process. Dry gangrene progresses relatively slowly, giving doctors time to attempt revascularization, a procedure to restore blood flow through the blocked artery. When endovascular therapy follows the anatomical map of which artery feeds which zone of the foot, studies have shown it can promote ulcer healing and reduce the rate of amputation.3PubMed. The Use of the Angiosome Concept for Treating Infrapopliteal Critical Limb Ischemia through Interventional Therapy and Determining the Clinical Significance of Collateral Vessels
Wet gangrene is far more dangerous. It develops when an infection takes hold in tissue that already has poor blood supply. The toe may be swollen, oozing, foul-smelling, and darkening rapidly. Wet gangrene spreads fast and can become life-threatening within days. If your toe is black and the skin around it is warm, swollen, or leaking fluid, treat it as an emergency.
Diabetes and the Black Spot That Should Never Be Ignored
Diabetes creates a perfect storm for foot problems. Nerve damage (neuropathy) means you may not feel a blister, cut, or pressure sore forming. Arterial disease reduces the blood flow needed to heal those wounds. And elevated blood sugar feeds bacteria, making infections harder to fight. A diabetic foot can be broadly categorized into one dominated by neuropathy, where circulation is still adequate but sensation is gone, and one dominated by ischemia, where arterial blockage reduces blood flow and leads to rest pain, ulceration, and gangrene.4PubMed Central. Infections in the diabetic foot
In people with diabetes, even a tiny break in the skin can escalate quickly. The appearance of the first “black spot” on a diabetic foot has been described as the point where a minor skin break becomes a major infection with a grave prognosis, potentially leading to loss of a limb or even death.4PubMed Central. Infections in the diabetic foot That is not hyperbole meant to frighten. It reflects the reality that infection in an ischemic foot does not stay local the way it might in a foot with good circulation.
One underappreciated problem is delay. Research on why people with diabetes postpone treatment for foot ulcers has identified a cluster of psychological barriers including fear of hospitalization or amputation, depression and anxiety, denial, and low social support.5PubMed Central. Treatment Delay in Patients With Diabetes‐Related Foot Ulcers: A Walker and Avant Concept Analysis These are understandable human reactions, but they can turn a salvageable situation into an irreversible one. If you have diabetes and notice any dark discoloration on your toes, the window for effective treatment is narrower than you think.
Preventing Diabetic Foot Disasters
International guidelines recommend that people with diabetes at very low risk of foot ulcers be screened annually for loss of protective sensation and peripheral artery disease, with higher-risk individuals screened more frequently. Prevention also includes education on foot self-care, never walking barefoot or without suitable foot protection, and promptly treating any pre-ulcerative lesion. For people at moderate to high risk, properly fitting therapeutic footwear that relieves pressure on the sole can help prevent ulcer recurrence.6PubMed. Guidelines on the prevention of foot ulcers in persons with diabetes (IWGDF 2023 update)
Daily foot checks are the single most actionable habit. Look between and under the toes, check for color changes, temperature differences, and any breaks in the skin. A hand mirror helps if flexibility is limited. The point is to catch problems before they become black spots.
Frostbite and Cold Injury
Exposure to extreme cold can turn toes black in a different way. Frostbite damages tissue through two distinct mechanisms: first, ice crystals form inside cells, physically tearing them apart and blocking blood flow; second, when the tissue rewarms, the inflammatory and clotting response triggered by reperfusion causes further damage.7PubMed Central. Frostbite: diagnosis, treatment, prognosis, and future directions The blackening of frostbitten toes usually appears after rewarming, once it becomes clear which tissue has died.
Severe frostbite often looks worse before the final extent of damage is clear, which is why the old surgical maxim of “freeze in January, amputate in July” exists. Doctors prefer to wait weeks before deciding how much tissue needs to come off, because some areas that initially look dead may recover. Immediate treatment focuses on rapid rewarming in warm water and preventing refreezing, which causes even more destruction than the original freeze. If your toes turned black after cold exposure, do not try to rub them or warm them near a direct heat source. Seek medical care for controlled rewarming.
Cholesterol Crystal Emboli and Blue Toe Syndrome
Sometimes a toe turns blue, purple, or black while the pulses in the foot are still present and strong. Blue toe syndrome occurs when tiny cholesterol crystals break free from an atherosclerotic plaque upstream, travel through the bloodstream, and lodge in the small arteries feeding the toes.8PubMed Central. Blue toe syndrome – systemic cholesterol crystal embolism secondary to cardiovascular procedures: a forensic autopsy report of two cases The toe becomes ischemic not because the main artery is blocked, but because its tiny downstream branches are clogged with debris.
This condition can happen spontaneously, but it is also a recognized complication of vascular procedures such as catheterization or aortic surgery, where manipulating diseased vessels can dislodge plaque. The key clue is that the foot may feel warm and have palpable pulses, yet one or two toes are dark and painful. If you recently had a cardiac or vascular procedure and a toe turns dark, bring it to your doctor’s attention immediately. The discoloration can progress from blue-purple to black if the tissue dies.
Buerger’s Disease in Younger Smokers
Peripheral artery disease from atherosclerosis mostly affects older adults, but there is a separate inflammatory vascular disease that strikes younger people, almost exclusively smokers. Buerger’s disease (thromboangiitis obliterans) inflames and clots the small and medium arteries and veins of the hands and feet. The clinical criteria include being under 45, current or recent tobacco use, and signs of ischemia in the fingers or toes such as pain at rest, ulcers, or gangrene.9PubMed Central. Thromboangiitis obliterans (Buerger’s disease)
If you are a younger smoker and a toe is turning dark, this diagnosis should be on the table. The condition is not atherosclerosis and does not respond to the same treatments. The single most effective intervention is complete tobacco cessation, including smokeless tobacco and, increasingly, vaping products. People who quit can often halt the progression. People who keep smoking face a high probability of amputation.
Subungual Melanoma
This is the cause people fear most and the one most commonly missed. Melanoma can develop under a toenail, producing a dark streak or patch of pigment that grows slowly over weeks to months. It has nothing to do with sun exposure; subungual melanoma arises from melanocytes in the nail matrix itself. It is uncommon overall but accounts for a disproportionate share of melanoma diagnoses in people with darker skin tones, partly because the more common sun-related forms are rarer in this group.
Dermatologists use a clinical detection framework sometimes called the ABCDEF rule for subungual melanoma. The key warning signs include a band of pigment that is brown to black and 3 mm or wider, with irregular or variegated borders; change in the nail band over time, or failure of a dark spot to improve despite treatment; involvement of the thumb or great toe (the most commonly affected digits); extension of pigment onto the skin of the nail fold, known as Hutchinson’s sign; and a personal or family history of melanoma.10PubMed. The ABC rule for clinical detection of subungual melanoma
The practical takeaway: a dark line or smudge under a toenail that was not caused by a clear injury, especially if it has been slowly changing, should be evaluated by a dermatologist. Subungual melanoma has a worse prognosis than other melanoma types largely because it gets diagnosed later. People assume it is a bruise, fungal infection, or normal pigmentation, and by the time a biopsy is done, the cancer has advanced.
Bacterial and Fungal Infections That Darken the Nail
Not every dark toenail is blood, melanoma, or gangrene. Certain infections can mimic a black toe convincingly. Pseudomonas aeruginosa, a bacterium that thrives in moist environments, can colonize a nail that has already lifted from its bed (a condition called onycholysis). The pigments it produces can turn the nail blue-black or green.11JAMA Dermatology. The Role of Pseudomonas Aeruginosa in Infections About the Nails The color may range from pale green to nearly black depending on the severity and how much pigment has accumulated.12PubMed. Green nail: Etiology and treatment of chloronychia
Fungal nail infections (onychomycosis) can also darken a nail to brown or black, though more commonly they produce white, yellow, or chalky discoloration. The clue with fungal infections is that they develop gradually, affect the texture and thickness of the nail, and are usually not painful unless the nail becomes very deformed. Treatment with topical or oral antifungals resolves the color change, though it takes months because you are waiting for a healthy nail to grow in.
The key distinction from melanoma is that infections tend to affect the nail plate diffusely or start at the edges, while melanoma produces a longitudinal streak in the nail. Still, the overlap in appearance is real enough that a suspicious dark nail deserves a professional opinion.
Less Common Causes Worth Knowing About
A few rarer conditions round out the list of things that can blacken a toe:
- Calciphylaxis: This condition involves calcium deposits in the walls of small blood vessels, leading to painful skin necrosis. It is most common in people with end-stage kidney disease on dialysis, but it can occur in people with other conditions that disturb calcium and phosphate balance. Skin biopsies in calciphylaxis patients show fibrin clots in small vessels, and the presence of these clots has been linked to worse pain.13PLOS ONE. Correlation between clinical and pathological features of cutaneous calciphylaxis The resulting skin lesions are darkly discolored, exquisitely painful, and prone to infection.
- Warfarin-induced skin necrosis: Blood thinners like warfarin can, paradoxically, cause clotting in small skin vessels during the first days of treatment, especially in people with protein C or protein S deficiency. The result is purplish-black patches of dying skin, sometimes on the toes.
- Toe-tourniquet syndrome: In infants, a hair or thread fiber can wrap tightly around a toe, cutting off circulation. The toe swells, reddens, and can turn dark. It is easily missed because the constricting strand can be buried in a skin crease. One case report described a four-month-old whose second and third toes were swollen and red, with a hair fiber protruding through the wound; the treatment was simply cutting and removing the constricting hair.14PubMed Central. Toe-tourniquet syndrome: A rare potentially devastating entity If a baby is crying inconsolably and you cannot figure out why, check the toes.
When to Worry and When to Wait
A few rules of thumb help sort the urgent from the merely concerning:
- Clear trauma, pain under the nail, no other symptoms: Likely a subungual hematoma. You can wait a day or two. If pressure is severe, see a doctor for drainage. If the nail lifts off entirely, keep it clean and watch for infection.
- No trauma, gradually darkening streak or spot: Could be melanoma, a fungal infection, or normal ethnic pigmentation. See a dermatologist within a few weeks. Do not assume it will go away.
- Cold, numb, or painful toe with progressive darkening: Suggests compromised blood flow. This warrants same-day or next-day evaluation, especially if you have risk factors for vascular disease.
- Diabetes plus any dark spot on the foot: Seek care within 24 hours. The threshold for concern in a diabetic foot is much lower than in someone with normal circulation and sensation.
- Swelling, warmth, odor, or oozing alongside the dark color: Possible wet gangrene or serious infection. Go to an emergency department.
- Recent vascular procedure plus a newly discolored toe: Possible cholesterol embolism. Contact your surgical team immediately.
Hyperbaric Oxygen and Emerging Treatments for Severe Cases
When a darkened toe is part of a larger wound-healing crisis, especially in people with diabetes, treatment goes beyond antibiotics and revascularization. Hyperbaric oxygen therapy, which involves breathing pure oxygen in a pressurized chamber, has been explored as an add-on treatment for chronic diabetic foot ulcers that do not respond to standard wound care. Case evidence suggests that combining hyperbaric oxygen with modern wound-care strategies may speed healing and reduce the risk of amputation in selected patients.15PubMed Central. Adjunctive hyperbaric oxygen therapy for chronic diabetic foot ulcer unresponsive to standard care: A case report The evidence is still limited to small studies and case reports, and it is not a first-line therapy, but it represents an option when conventional treatment has stalled and amputation is being considered.
Other areas of active research include advanced wound dressings that deliver growth factors directly to the wound bed, negative-pressure wound therapy (sometimes called wound vacs), and stem-cell-based approaches to promote blood vessel growth in ischemic tissue. None of these has displaced the fundamentals of good diabetic foot care: controlling blood sugar, maintaining blood flow, preventing infection, and offloading pressure from the wound. But for people facing the prospect of losing a toe or more, the expanding treatment toolkit offers reasons for cautious optimism.