A blue toenail is most often a subungual hematoma, which is a pocket of blood trapped between the nail plate and the nail bed after some kind of injury. The bruise-like discoloration can look alarming, but in the majority of cases it grows out on its own over weeks to months. That said, blue discoloration under a toenail can also signal circulatory problems, medication side effects, infections, or, in uncommon cases, tumors beneath the nail. Sorting out which cause applies to you depends on a handful of details: whether there was an obvious injury, whether the color appeared suddenly or gradually, and whether you have pain, coldness, or other symptoms alongside the color change.
Trauma Is the Most Common Cause
Stubbing your toe, dropping something on your foot, or wearing shoes that are too tight can all rupture tiny blood vessels beneath the toenail. The blood has nowhere to drain, so it pools under the nail plate and shows through as a dark blue, purple, or black patch. Doctors call this a subungual hematoma, and it accounts for the vast majority of single-nail blue discolorations.1PubMed Central. Subungual Hematoma: Insights From a Clinical Case Study The color comes from hemoglobin breaking down as the trapped blood ages, which is why the spot can shift from bright red-purple to dark blue or brownish-black over the following days.
The pain is usually the first thing people notice. Because the nail plate is rigid, the expanding pocket of blood creates pressure against the sensitive nail bed underneath. That pressure can be intense enough to throb with each heartbeat. Prompt drainage of the hematoma through a small hole in the nail, a procedure called nail trephination, gives almost immediate relief.2The Journal of Emergency Medicine. Use of an 18-Gauge Needle for Evacuation of Subungual Hematoma If the hematoma is small and pain is manageable, many people simply let the discolored nail grow out on its own, which can take several months for a toenail.
You do not always need a dramatic injury for this to happen. Repetitive microtrauma from long-distance running, hiking, or wearing ill-fitting shoes can cause the same pooling of blood over time. Runners sometimes call it “runner’s toe” or “black toenail.” The second toenail is a frequent target because it tends to strike the front of the shoe with each stride. In these cases the color change develops gradually rather than overnight, and you might not even remember a specific injury.
Cold Exposure and Raynaud’s Phenomenon
If your toenail turns blue mainly when you are cold and returns to normal color once you warm up, the issue is more likely circulatory than structural. Exposure to cold temperatures causes blood vessels in the fingers and toes to constrict, reducing blood flow. In most people the effect is mild and temporary, but in those with Raynaud’s phenomenon, the constriction is exaggerated and episodic, producing a striking sequence of color changes: the digits turn white as blood flow cuts off, then blue as the remaining blood loses oxygen, then red as circulation returns.3PubMed. Raynaud’s phenomenon and related vasospastic disorders
Raynaud’s is more common than many people expect, and prevalence varies by climate. It affects the fingers more often than the toes, but the toes are far from immune. In its primary form, the vasospasm is an overreaction to cold or emotional stress but is not connected to any underlying disease. Secondary Raynaud’s, however, is linked to autoimmune conditions such as lupus, scleroderma, and rheumatoid arthritis. The color changes themselves are reversible and not dangerous in primary Raynaud’s, but if attacks are frequent, severe, or accompanied by sores on the digits, a doctor should evaluate for an underlying autoimmune trigger.
Vascular Problems Beyond Raynaud’s
A persistently blue toe that does not warm up or improve on its own can point to a more serious vascular issue. Blue toe syndrome is a broad clinical term for cyanosis or reduced blood flow in the toes caused by partial or complete blockage of small and medium arteries. Causes include small blood clots that travel from a larger artery and lodge downstream, clotting disorders that make the blood more prone to coagulation, and cholesterol plaque fragments that break off from the walls of larger vessels.4PubMed Central. Blue Toe Syndrome: A Challenging Diagnosis
What makes blue toe syndrome tricky is that the larger pulses in the foot can feel completely normal. The blockage is happening farther downstream, in vessels too small to palpate. The toe may be painful, cool to the touch, and persistently discolored even when the rest of the foot looks fine. This is a medical situation that warrants prompt evaluation, because untreated obstruction can lead to tissue damage. It tends to occur in older adults with cardiovascular risk factors like high blood pressure, diabetes, or a history of smoking, and can also appear after vascular procedures or the start of blood-thinning medication.
Medications That Discolor Nails
Several prescription drugs can turn toenails blue, blue-gray, or slate-colored without any injury or vascular problem at all. Minocycline, a tetracycline-class antibiotic commonly prescribed for acne, is one of the best-documented offenders. It can deposit pigment in the skin, mucous membranes, and nail beds, producing a blue-gray discoloration near the nail bed that persists as long as the drug is taken and sometimes for months afterward.5PubMed Central. A novel presentation of minocycline-induced nail discoloration: A case report Other medications associated with bluish nail changes include certain chemotherapy agents, antimalarials like hydroxychloroquine, and some antiretroviral drugs.
The mechanism differs by drug. Minocycline forms pigmented complexes that are deposited directly in tissue. Some chemotherapy drugs cause the nail matrix cells to produce melanin irregularly. In either case, the discoloration tends to affect multiple nails rather than just one, which is an important clue. If you have recently started a new medication and notice your toenails changing color symmetrically across both feet, mention it to your prescriber. The change is typically cosmetic and reversible once the drug is stopped or switched, though it can take time for new, unaffected nail to grow in.
Infections and Unusual Colors
Bacterial infections of the nail can produce green, blue-green, or dark discoloration that gets mistaken for a bruise. The most common culprit is Pseudomonas aeruginosa, a bacterium that thrives in moist environments and produces a distinctive blue-green pigment called pyocyanin. When Pseudomonas colonizes the underside of a nail that is already lifted or loosened from the bed, it creates what clinicians call green nail syndrome or chloronychia.6PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons The nail looks green or greenish-blue, and the surrounding skin may be swollen without being particularly tender.
People whose feet are frequently damp or who have pre-existing nail damage are most susceptible. Elderly individuals and those with occupations involving prolonged water exposure are at higher risk. Fungal infections, by contrast, more often cause yellow, white, or brownish nails, but advanced fungal infections can sometimes darken nail color enough to look bluish under certain lighting. The treatment for Pseudomonas nail infection typically involves keeping the area dry, trimming away the separated nail, and applying topical antiseptics. Oral antibiotics are reserved for stubborn cases.
Glomus Tumors and Other Growths Under the Nail
When blue discoloration appears in a single toenail without any history of trauma, without cold exposure, and without medication use, the list of possibilities narrows to a few less common causes. A literature review of blue nail discoloration found that when only one nail is affected, the cause is most often a tumor or benign growth beneath the nail, with glomus tumors being the most frequent, followed by blue nevi and, less commonly, melanomas.7PubMed. Blue Nail Discoloration: Literature Review and Diagnostic Algorithms
A glomus tumor is a small, benign growth of the specialized smooth-muscle cells that regulate blood flow in the fingertips and toes. It classically presents with a triad of symptoms: intense pain at a very specific point, extreme sensitivity to cold, and point tenderness so precise you can sometimes pinpoint the tumor with the tip of a pen. A bluish or reddish spot visible through the nail plate is another hallmark.8The Journal of Foot and Ankle Surgery. Glomus tumor of the foot: A case report and review of literature Glomus tumors are benign, but they can cause years of unexplained pain if not identified. MRI is useful for confirming the diagnosis, and surgical removal generally resolves the symptoms completely.9PubMed Central. Hand and Foot Glomus Tumors: Significance of MRI Diagnosis Followed by Histopathological Assessment
Blue nevi, which are benign collections of pigment-producing cells deep in the skin, can also show up under the nail as a small blue-black spot. They are typically painless and stable over time. Melanoma of the nail unit is the rarest of these causes, but it is the most consequential to rule out.
How Doctors Tell a Bruise From Something Worse
From the outside, a subungual hematoma, a glomus tumor, a blue nevus, and an early melanoma can all look like a dark blue or black spot under the nail. The good news is that a technique called dermoscopy, which uses a magnifying lens and polarized light, can usually distinguish them without any cutting. When clinicians examined subungual hematomas with dermoscopy, a large majority showed a homogeneous purple-black color with a telltale fading at the edges, where the blood thins out as it spreads.10British Journal of Dermatology. Dermoscopy of subungual haemorrhage: its usefulness in differential diagnosis from nail‐unit melanoma The same study found that most hematomas showed combinations of more than one color, and that a globular pattern was common.
Melanoma of the nail unit, by contrast, displays features that are essentially never found in a simple hematoma. A key sign is the Hutchinson sign, where dark pigment extends from under the nail plate onto the surrounding skin fold. Irregular dark bands running lengthwise along the nail, widening as they approach the cuticle, are another red flag. In a series of nail-unit melanomas, irregular longitudinal bands appeared in most cases, while ulceration and the Hutchinson sign were present in all.10British Journal of Dermatology. Dermoscopy of subungual haemorrhage: its usefulness in differential diagnosis from nail‐unit melanoma None of these melanoma features were found in the hematoma cases.
There is also a simple home test, though it is not definitive: a subungual hematoma moves forward as the nail grows. If you mark the leading edge of the discoloration with a pen and check again in a few weeks, a hematoma should have migrated toward the tip of the toe. A melanoma or other fixed lesion does not migrate because the pigment is being produced by cells in the nail matrix, not by old blood passively riding the nail as it grows. This test is not a substitute for a medical evaluation, but it can provide some peace of mind while you wait for an appointment.
Nail Polish and Cosmetic Staining
Before assuming the worst, consider whether your blue toenail might simply be stained. Dark-colored nail polishes, especially those containing red and yellow pigments, are known to stain the keratin of the nail plate. The staining effect is stronger with polishes in which the pigments are dissolved rather than suspended in the formula.11CosmoDerma. Adverse effects of nail cosmetics and how to prevent them While the most common cosmetic stain color is yellow or orange-brown from red pigments, dark blues and purples from deeply pigmented polishes can leave behind a bluish residue on the nail after the polish is removed. Applying a base coat before colored polish and avoiding leaving dark polish on for extended periods reduces the risk.p>
The distinguishing feature of cosmetic staining is that it is superficial. It affects the top layers of the nail plate, not the tissue underneath. Gently buffing the nail surface may lighten the discoloration, and it grows out with the nail. There is no pain, no tenderness, and no associated symptoms. If you had dark nail polish on recently and the blue color appeared only after removing it, cosmetic staining is the likeliest explanation.
When to See a Doctor
Most blue toenails from obvious trauma resolve on their own, and most cold-triggered color changes in otherwise healthy people are benign. But certain patterns warrant medical attention sooner rather than later:
- No clear cause: Blue discoloration that appears without any injury, cold exposure, or new medication deserves evaluation, especially if it is limited to one nail.
- Persistent or worsening color: A bruise that does not migrate forward with nail growth over several weeks, or that darkens or expands rather than fading, should be examined.
- Pigment spreading to surrounding skin: Dark color extending from under the nail onto the cuticle or surrounding skin fold is a warning sign for melanoma and should be assessed urgently.
- Severe or disproportionate pain: A tiny blue spot that causes pain out of proportion to its size, especially pain triggered by cold or precise pressure, could indicate a glomus tumor.
- Signs of poor circulation: A persistently cold, blue toe with numbness or tingling that does not resolve with warming may indicate a vascular blockage.
- Multiple nails affected symmetrically: Blue-gray changes across several toenails suggest a systemic cause like a medication side effect or, less commonly, a metabolic condition.
A dermatologist can often reach a diagnosis just by looking at the nail with dermoscopy.12PubMed Central. Dermoscopic diagnosis of subungual hematoma: new observations If there is any concern about melanoma, a biopsy of the nail matrix is the definitive test. For suspected vascular problems, your doctor may check pulses in the foot and order imaging of the blood vessels. The threshold for seeking care should be low if you are over 50, have cardiovascular risk factors, or have any history of melanoma.
Why Toenails Are Diagnosed Later Than Fingernails
Nail melanoma and glomus tumors are diagnosed significantly later in the toes than in the fingers, and the reason is mostly behavioral. People look at their fingernails all day long; they rarely inspect the undersides of their toes. Shoes and socks hide discoloration for months. Toenails also grow much more slowly than fingernails, so a subungual hematoma that would grow out in a couple of months on a finger can linger on a toe for six months to a year, making it harder to tell whether the color is migrating or stationary.
This delayed recognition matters because subungual melanoma, the most serious cause of a dark nail, has a worse prognosis when diagnosed at an advanced stage. Melanoma of the nail unit is uncommon in the general population but is disproportionately represented in people with darker skin tones, for whom it is one of the more common melanoma subtypes. Getting in the habit of checking your toenails when you trim them, and treating any new, unexplained dark streak under a toenail the way you would treat a new mole on your skin, is a simple way to catch problems earlier.