A black toenail is one of those symptoms that sends people immediately to the worst-case scenario. Melanoma. Cancer. Something serious. In the vast majority of cases, the reality is considerably less alarming: blood has pooled under the nail from trauma, the nail looks dramatic, and in a few weeks it grows out entirely.
But sometimes it is more than that. Knowing which situation you are in is genuinely important, and the good news is that the distinguishing features are fairly clear once you know what to look for.
This guide walks through the four main causes of black toenails, how to tell them apart, and the specific signs that mean you should see a doctor rather than wait it out.
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. If you are concerned about a toenail or any other symptom, consult a qualified healthcare professional.
Cause 1: Subungual Hematoma (Blood Under the Nail)
This is responsible for the overwhelming majority of black toenails and is almost always benign. A subungual hematoma occurs when blood pools between the nail plate and the nail bed, usually following direct trauma or repetitive pressure.
Two common patterns:
Acute trauma is the obvious one: you drop something on your toe, kick a hard surface, or stub it badly. The blood pools rapidly, pressure builds under the nail, and the affected area turns dark red, purple, or black within hours. This type is often painful immediately after the injury.
Repetitive microtrauma is subtler and more commonly affects runners, hikers, and people who wear shoes that are slightly too short or too tight. The toe repeatedly makes contact with the front of the shoe during downhill movement or sustained effort. The blood accumulates over time rather than suddenly. Many runners notice their first black toenail after a long race or a particularly hilly training run.
How to recognize a subungual hematoma:
- There is a clear history of injury or physical activity that explains it
- The discoloration appeared shortly after the incident or activity
- The dark area moves forward as the nail grows over weeks
- There may have been pain initially, which fades
- The nail itself looks structurally normal except for the color
Treatment is usually none at all. The nail grows out, the discolored portion reaches the free edge, and you trim it away. In cases where pressure under the nail is causing significant pain, a doctor can relieve it with a simple procedure called trephination (creating a small opening to drain the blood), but this is not medically necessary in most cases.
Cause 2: Fungal Infection (Onychomycosis)
Fungal nail infections are the second most common cause of discolored toenails overall, though they more typically produce yellow, brown, or whitish nails rather than pure black. When debris and fungal material accumulate under a nail with a heavy infection, the result can look very dark.
Onychomycosis is caused most commonly by dermatophytes, the same group of fungi responsible for athlete’s foot. The infection spreads from the skin to the nail and progresses slowly over months to years without treatment.
How to recognize a fungal nail infection:
- The nail is thickened and harder to trim than normal
- The texture is brittle or crumbly rather than smooth
- There may be a mild unpleasant odor
- Adjacent nails or the skin between toes show signs of athlete’s foot
- The discoloration started at the free edge and moved toward the base over time
- Multiple nails are often affected
Treatment requires patience. Over-the-counter antifungal treatments applied topically have limited effectiveness for established nail infections because they cannot penetrate the nail plate reliably. Prescription oral antifungals (terbinafine, itraconazole) are more effective but require a course of several months and are not appropriate for everyone due to potential interactions. A podiatrist or dermatologist can culture the nail to confirm the specific organism and recommend appropriate treatment.
Cause 3: Subungual Melanoma
This is the serious one, and it is also the rare one. Subungual melanoma accounts for roughly 0.7% to 3.5% of all melanomas in lighter-skinned populations, with higher rates in people of African, Asian, and Hispanic descent (where it represents a larger proportion of total melanoma cases).
It typically presents as a dark longitudinal streak running the length of the nail (called melanonychia striata) rather than a uniform dark area. It can look very similar to a subungual hematoma, which is part of why it is sometimes diagnosed late.
The feature that most reliably distinguishes subungual melanoma from other causes is called the Hutchinson sign: pigmentation extending from the nail bed into the surrounding cuticle or skin at the nail fold. If you see darkness spreading onto the skin around the nail, not just under it, this warrants immediate medical evaluation.
Other warning signs for subungual melanoma:
- Dark streak or spot with no history of injury that explains it
- The discoloration does not grow out as the nail grows
- The streak widens over time or develops irregular borders
- Multiple colors within the discolored area (browns, blacks, grays)
- The nail begins to split, crack, or separate from the nail bed
- Pigment extends to surrounding skin (Hutchinson sign)
Cause 4: Other Medical and Systemic Causes
Less commonly, black or very dark toenails can indicate other conditions worth knowing about:
Pseudomonas bacterial infection typically causes a blue-green or dark discoloration (sometimes called “green nail syndrome”) and is associated with nails that have been damaged or that retain moisture. It is more common in people who frequently have wet hands or feet.
Medication side effects including certain chemotherapy agents, antimalarials, and some antibiotics can cause nail pigmentation as a side effect. If you have started a new medication and noticed nail changes, it is worth mentioning to your prescribing physician.
Psoriasis can affect the nails in multiple ways, including causing dark discoloration from oil-drop (salmon patch) lesions beneath the nail. Other signs of nail psoriasis include pitting, ridging, and separation of the nail from the nail bed.
Systemic conditions including poorly controlled diabetes and cardiovascular disease can manifest in nail changes, generally as part of a broader pattern of symptoms rather than as an isolated finding.
Quick Reference: How to Tell Them Apart
| Feature | Hematoma | Fungal | Melanoma |
|---|---|---|---|
| History of injury | Yes, usually clear | No | No |
| Grows out with nail? | Yes | Spreads / persists | No |
| Nail texture | Normal | Thick, crumbly | May split or lift |
| Spreads to skin? | No | Adjacent skin only | Yes (Hutchinson sign) |
| Multiple nails? | Only if multiple traumas | Often | Rarely |
| Pain | Initially, then fades | Mild or none | Variable |
When to See a Doctor
The following situations warrant a visit to a podiatrist or dermatologist rather than a wait-and-see approach:
- You have no memory of any injury or physical activity that could have caused the blackening
- The dark area has not moved forward after 4 to 6 weeks of nail growth
- Pigmentation is extending onto the surrounding skin (Hutchinson sign)
- The discolored area is widening or changing shape
- The nail is lifting from the nail bed, cracking, or separating
- You have diabetes, a compromised immune system, or circulatory problems in your feet
- Pain, swelling, or any discharge is present
For most people reading this, the black toenail has a mundane explanation and will resolve on its own. But the subset of cases that do not fit the trauma or fungal pattern deserve a professional look. Subungual melanoma has a much better prognosis when caught early, and the evaluation itself is straightforward. When in doubt, get it checked.
