Dark toenails most often result from blood pooling under the nail after some kind of physical impact, a condition called a subungual hematoma. This is the single most common explanation, and it ranges from obvious (you dropped something on your foot) to surprisingly subtle (your shoe rubbed your longest toe on every step of a long run). But blood under the nail is far from the only possibility. Fungal infections, certain medications, pigment-producing cells in the nail bed, and even rare tumors can all turn a toenail dark, and distinguishing between a harmless bruise and something that needs medical attention matters more than most people realize.
Trauma and Subungual Hematomas
The most frequent reason a toenail turns dark blue, purple, or black is a subungual hematoma, which is simply trapped blood between the nail plate and the nail bed underneath it. This can happen from a single acute event, like stubbing your toe or dropping a heavy object, or from repetitive low-grade pressure over time. Runners, hikers, and people who play sports with frequent starts and stops are especially prone to the repetitive type, where the toe slides forward inside the shoe with each stride and the shearing force gradually damages tiny blood vessels underneath the nail.1PubMed. Managing Toenail Trauma
A fresh subungual hematoma usually appears red or dark purple and can be quite painful because of the pressure from trapped blood. Over days and weeks, the color shifts to dark blue or black as the blood breaks down. The discoloration then slowly migrates toward the tip of the toe as the nail grows out, which can take several months for a toenail. In some cases the nail lifts off entirely and a new one grows in behind it. If you can clearly remember an injury or you have been breaking in new shoes, this is almost certainly what you are dealing with.
A related phenomenon is splinter hemorrhages, which look like thin reddish-brown or brown streaks running lengthwise under the nail. A study of toenail splinter hemorrhages in adults found that over 90% occurred in the big toenails, and about 60% of patients could identify a clear triggering event like an injury or tight footwear. Under magnification, these hemorrhages typically show a reddish-brown stain with a clear boundary that fades toward the edges, with no blue or white structures underneath.2PubMed Central. Clinical Observation and Follow-Up of Subungual Splinter Hemorrhages of Toenails in Chinese Adults If you notice thin dark lines in your big toenail and recently hiked in snug boots or played a sport, splinter hemorrhages are the likely culprit.
Fungal Infections That Darken Nails
Fungal toenail infections, collectively called onychomycosis, are probably the second most common reason toenails turn dark. Most people picture yellowed, thickened nails when they think of nail fungus, and that is typical of the usual culprits (dermatophyte fungi). But a specific group of fungi called dematiaceous or phaeoid molds produces melanin in their cell walls, a brown-to-black pigment that stains the nail plate dark rather than yellow. These fungi generate dark-colored colonies and structures that can make the nail look brown, greenish-black, or even jet black.3PubMed Central. Onychomycosis caused by dematiaceous fungi: A four-year study on agricultural workers of Assam, India
The giveaway for fungal involvement is usually the texture of the nail. A nail that is only darkened from trapped blood stays relatively smooth and attached to the bed, while a fungal nail tends to become thickened, crumbly, or chalky along with the color change. If your toenail is dark and also distorted, lifting away from the nail bed, or has a powdery or flaky surface, a fungal infection is worth investigating. Diagnosis is straightforward: a doctor can clip a sample and send it for culture or microscopic examination, and treatment is typically an oral or topical antifungal medication depending on severity.
Bacterial Infections and Green-Black Nails
Bacteria can also cause darkened nails, most notably Pseudomonas aeruginosa, which produces pigments that turn the nail plate greenish-yellow, greenish-brown, or greenish-black. This is sometimes called green nail syndrome or chloronychia. It tends to develop in nails that are already compromised, particularly in people whose hands or feet are frequently wet or who have existing nail damage. The infection usually involves the space between the nail plate and the nail bed, and the discoloration can range from a subtle olive tint to a dramatic dark green or black.4PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons
Pseudomonas infections are more common in fingernails than toenails, but they do occur in the feet, especially in older adults. The green or greenish-black color is distinctive enough that it often suggests the diagnosis on its own. Treatment usually involves an antibiotic, and keeping the nail dry is an important part of preventing recurrence.
Longitudinal Melanonychia and Pigmented Streaks
If the dark color on your toenail takes the form of a brown or black stripe running from the base of the nail to the tip, you are likely looking at longitudinal melanonychia. This is not a single disease but a pattern: melanocytes (the pigment-producing cells that also give skin its color) in the nail matrix become activated and deposit melanin into the growing nail, creating a visible band. The causes range from completely harmless (friction, normal ethnic variation, hormonal changes) to potentially dangerous (melanoma of the nail bed).
Skin color plays a significant role in how melanonychia presents and how clinicians evaluate it. Research shows that people with darker skin tones tend to have wider and darker pigmented bands compared to those with lighter skin, and their bands are more likely to show brown coloration rather than gray under dermoscopy. These features can mimic warning signs for melanoma, which leads to higher biopsy rates in darker-skinned patients even though their bands are often benign.5PubMed. Clinical and dermoscopic findings of benign longitudinal melanonychia due to melanocytic activation differ by skin type and predict likelihood of nail matrix biopsy If you have darker skin and notice a brown or black stripe on a toenail, it is reasonable to have it checked, but it is also helpful to know that such streaks are far more common and usually benign in people with more melanin in their skin.
When Dark Nails Could Be Melanoma
The cause everyone fears, and the reason you should not simply ignore a persistent dark mark on a toenail, is subungual melanoma. This is melanoma arising in the nail bed or nail matrix, and it is genuinely rare, accounting for a small fraction of all melanoma cases. But it is disproportionately dangerous because it tends to be caught late: people assume the dark spot is a bruise or fungus and delay seeing a doctor until the cancer has advanced.
One key clinical clue is the Hutchinson sign, where pigment spreads from the nail plate onto the surrounding skin of the nail fold or fingertip. A study examining this sign found that it appeared most often on the skin underneath the free edge of the nail (the hyponychium), with irregular diffuse pigmentation visible in every case examined. Under magnification, a pattern of parallel ridges was seen in about 60% of cases, and biopsies confirmed scattered atypical melanocytes with abnormal nuclei in every specimen.6PubMed. Hutchinson Sign: Biopsy May Assist in Diagnosis of Subungual Melanoma in Situ
The practical warning signs worth remembering form a rough checklist. A dark band that is wider than about three millimeters, has blurry or irregular borders, changes in width or color over weeks to months, or shows pigment bleeding onto the skin surrounding the nail warrants a visit to a dermatologist. Subungual melanoma is more common on the big toe and thumb than other digits. If you have a dark spot under a toenail and it does not grow out with the nail over a few months the way a bruise would, that alone is worth getting checked. The key difference: a bruise migrates forward as the nail grows and eventually disappears, while melanoma-related pigment stays anchored in the same spot or expands.
How Doctors Tell the Difference
Distinguishing a harmless subungual hematoma from a melanocytic lesion is not always easy with the naked eye, and this is where dermoscopy comes in. A dermatoscope is essentially a magnifying lens with a specialized light source that lets a clinician see structures beneath the nail plate that are invisible to the unaided eye. Research has found that combining dermoscopy with optical coherence tomography, a type of high-resolution imaging, significantly improves the ability to differentiate hematomas from melanocytic lesions under the nail.7PubMed. Examination of Subungual Hematomas and Subungual Melanocytic Lesions by Using Optical Coherence Tomography and Dermoscopy
In practice, your dermatologist will look for specific patterns. A hematoma typically shows a homogeneous reddish-brown or dark red patch that fades at the edges and sometimes has a blood-blister-like globular appearance. Melanocytic lesions, by contrast, tend to show organized bands or irregular patches of brown and black with distinct structural patterns at the ridge level. If the dermoscopic exam raises suspicion, the next step is usually a biopsy of the nail matrix, which involves removing a small piece of the tissue that generates the nail. This is the definitive test, though it is not routine for every dark toenail.
Medications That Change Nail Color
Several classes of drugs can darken the nails as a side effect. Chemotherapy drugs are among the best-known culprits. Cancer patients undergoing chemotherapy frequently develop nail changes that range from darkened bands and overall discoloration to ridges, brittleness, and even nail loss. Many of these changes are cosmetic and resolve after treatment ends, but some can be painful or significantly affect daily life.8PubMed Central. Nail Changes With Chemotherapeutic Agents and Targeted Therapies
Outside of chemotherapy, antimalarial drugs like hydroxychloroquine, certain antibiotics (notably minocycline and some fluoroquinolones), antiretroviral medications used for HIV, and even some blood pressure drugs can all cause nail darkening. The discoloration typically develops gradually over weeks to months of use and affects multiple nails rather than just one. If your toenails have been turning dark since you started a new medication, mention it to your prescribing doctor. In most cases, the color reverts to normal after the drug is discontinued, though it can take the full time needed for a new nail to grow in, which is roughly 12 to 18 months for a big toenail.
Psoriasis, Lichen Planus, and Inflammatory Skin Conditions
Inflammatory diseases that affect the skin can also involve the nails, sometimes producing dark discoloration. Nail psoriasis is a common example. When psoriasis targets the nail bed, it can cause a characteristic set of changes including onycholysis (the nail lifting from the bed), salmon-colored or yellowish “oil drop” patches, and splinter hemorrhages, which are the thin dark streaks described earlier. Nail bed psoriasis can also produce general discoloration and thickening that makes the nail appear darker than normal.9PubMed Central. Nail psoriasis – what a rheumatologist should know about
Lichen planus is another inflammatory condition that can affect toenails, sometimes causing dark longitudinal ridges or overall darkening. These conditions are usually diagnosed based on the combination of nail changes, skin findings elsewhere on the body, and sometimes a biopsy. If you have a known inflammatory skin condition and notice toenail darkening, it is more likely related to the disease itself than to a separate problem, but confirming that with a dermatologist is still worthwhile.
Endocrine and Systemic Causes
Darkened toenails can sometimes be a sign of a systemic condition rather than a local nail problem. Addison’s disease, a condition in which the adrenal glands produce insufficient hormones, is a classic example. The hormonal imbalance stimulates melanocytes throughout the body, leading to hyperpigmentation of the skin and, sometimes, the nails. Nail darkening in Addison’s disease typically appears as melanonychia, and case reports have documented it occurring with a pseudo-Hutchinson sign, meaning pigment that appears to extend onto the surrounding skin but is actually visible through a translucent nail fold rather than growing into the skin itself.10PubMed Central. Melanonychia with pseudo-Hutchinson sign may assist in diagnosis of Addison’s disease This distinction matters because true Hutchinson sign raises concern for melanoma, while pseudo-Hutchinson sign points toward a systemic hormonal issue.
Other systemic conditions linked to nail hyperpigmentation include Cushing syndrome, hyperthyroidism, and certain nutritional deficiencies, particularly vitamin B12 and folate deficiency. In these situations, the nail changes tend to affect multiple nails on both hands and feet, which is a useful clue: if all your toenails are gradually darkening, a systemic cause is more plausible than a localized one. If only one toenail is involved, think local (trauma, fungus, tumor).
Glomus Tumors and Other Rare Growths
Occasionally, a small dark spot under a toenail turns out to be a glomus tumor, a benign growth arising from the glomus body, a structure involved in regulating blood flow and temperature in the fingertips and toes. These tumors are typically under one centimeter and appear as a faint blue-red spot beneath the nail. The hallmark symptom triad is localized tenderness, pain triggered by cold exposure, and sharp pain with even minor pressure on the nail.11PubMed Central. The hidden cause of chronic finger pain: Glomus Tumor – A Case Report If you have a dark spot under one toenail that has been tender for months and hurts particularly when it gets cold, mention glomus tumor to your doctor. They are benign and curable with surgical removal, but frequently go undiagnosed for years because they are not common enough to be on everyone’s radar.
Foot Structure and Shoe Fit
Your foot shape and biomechanics can predispose certain nails to repeated darkening. A study examining the relationship between foot abnormalities and nail changes found that common structural issues like bunions were strongly associated with nail problems including compression nails, ingrown nails, and fungal infections. Compression nails, which result from chronic pressure on the nail, were among the most frequently observed changes.12PubMed Central. Foot, Toe, and Nail Changes: Are They Interdependent? When a bunion pushes the big toe into an abnormal angle, the nail experiences chronic compression and shearing against the shoe, which can lead to recurrent subungual hematomas and darkening.
If you find that your toenails darken repeatedly despite wearing properly sized shoes and avoiding obvious trauma, consider having a podiatrist evaluate your foot structure. Morton’s toe (where the second toe is longer than the big toe), hammertoes, and high arches can all create abnormal pressure points that repeatedly damage the nail. Addressing the underlying biomechanical issue, sometimes with orthotics or a change in shoe style, can break the cycle.
Dark Toenails in Children
Parents who notice a dark streak on their child’s toenail understandably worry, especially if they search online and encounter information about melanoma. But the rules that apply to adults do not map neatly onto children. Pediatric longitudinal melanonychia has distinct clinical and histopathological features, and many of the warning signs used to evaluate adults, like band width and color irregularity, are unreliable in children. Children’s pigmented nail bands may look cytologically atypical under a microscope yet still follow a completely benign course.13PubMed Central. Longitudinal Melanonychia in Children: Clinical and Histopathologic Features and Management with Literature Update
Subungual melanoma is exceedingly rare in children. The most common causes of dark toenails in kids are trauma (children are not always aware of or honest about toe injuries) and benign melanocytic activation, which can produce a pigmented band that simply needs periodic monitoring rather than aggressive intervention. A pediatric dermatologist can provide age-appropriate guidance, but the threshold for alarm is much higher in a child than in an adult with the same finding.
Practical Steps When You Notice a Dark Toenail
The first question to ask yourself is whether you can link the darkening to an obvious cause. A recent injury, new shoes, a long hike, starting a new medication, or a known history of nail fungus all point you toward explanations that are common and usually manageable. Take a photo of the nail with your phone and date it. This gives you and your doctor a reference point to track whether the discoloration is growing out (suggesting a hematoma) or staying in place and changing (suggesting something that warrants closer examination).
Seek a dermatology evaluation if any of the following apply:
- No clear cause: You cannot identify any trauma, medication, or other explanation for the darkening.
- Pigment on surrounding skin: Dark color extends beyond the nail plate onto the skin of the toe.
- No growth-out over months: A bruise should migrate toward the tip of the nail; if the dark area stays fixed or expands, it needs evaluation.
- A single dark band: One dark longitudinal stripe on one nail is more concerning than uniform darkening across multiple nails.
- Pain without injury: Persistent tenderness, especially with cold or pressure, could suggest a glomus tumor or other lesion.
Resist the urge to self-treat by filing down or removing a darkened nail at home. If the cause turns out to be a melanocytic lesion, disrupting the nail can make diagnosis harder and, in rare cases, delay treatment for something serious. For a straightforward subungual hematoma that is painful and throbbing, a doctor can relieve the pressure by making a small hole in the nail plate, a quick in-office procedure that provides nearly immediate relief and allows the trapped blood to drain.
Why Multiple Darkened Nails Tells a Different Story
A useful mental shortcut: one dark toenail usually means a local problem, while several dark nails at once usually means something systemic. Local causes like trauma, a fungal infection in one nail, or a melanocytic lesion almost always affect a single nail or two. Systemic causes like medication side effects, Addison’s disease, B12 deficiency, or even pregnancy-related hormonal shifts tend to affect several or all nails simultaneously, and often the fingernails too.
If you notice that multiple toenails (or toenails and fingernails together) are gradually darkening, the cause is less likely to be anything requiring a biopsy and more likely to be something metabolic, hormonal, or pharmacological. Mention it at your next medical appointment. Basic bloodwork can screen for the most common culprits, and a medication review may reveal an obvious explanation hiding in your pill organizer.