A black spot on your toenail is almost always a bruise, a harmless patch of pigmentation, or a fungal infection. These three causes account for the vast majority of cases. Rarely, a dark spot can signal something more serious like melanoma, so knowing what to watch for matters.
A Bruise Under the Nail
The single most common cause of a black spot on a toenail is a bruise, sometimes called a subungual hematoma. When something injures the blood vessels in your nail bed (stubbing your toe, dropping something on it, wearing tight shoes), blood leaks and pools in the tiny space between your nail and the skin underneath. Because there’s nowhere for the blood to go, it gets trapped and shows through the nail as a dark spot.
The color typically starts out red or purple and gradually turns dark brown or black over days. You may or may not remember the injury that caused it. A key feature: the spot will slowly move toward the tip of your toe as your nail grows out. Toenails grow at roughly 1.6 millimeters per month, so it can take six to nine months for the discoloration to fully disappear. If the spot doesn’t move forward with normal nail growth, that’s a reason to get it checked.
Runner’s Toe and Repetitive Friction
You don’t need a single dramatic injury to bruise a toenail. Repetitive micro-trauma produces the same result. Runners, soccer players, tennis players, and rock climbers commonly develop black toenails from their feet repeatedly pressing or slamming into their shoes. It’s essentially like lightly hitting your toe over and over again until the small blood vessels underneath start to leak.
Prevention comes down to shoe fit. Leave about half an inch of space between your longest toe and the end of your shoe, and make sure you can freely wiggle your toes. Keeping toenails trimmed short also reduces the chance of them catching against the shoe. Sport-specific footwear tends to account for the directional forces your feet encounter during that activity.
Natural Pigmentation
Not every dark mark on a toenail is blood. Some people develop pigmented streaks or spots from the cells that produce melanin in the nail matrix, the tissue at the base of your nail where growth begins. This is called melanonychia, and it’s overwhelmingly benign.
Skin tone plays a major role. Among African Americans, somewhere between 77% and 100% develop some degree of nail pigmentation over their lifetime. The rate is 10% to 20% among Japanese and Asian populations, and about 1% in white populations. Nearly 20% of people of Japanese descent have benign melanonychia specifically. Among people of Caribbean descent, nail discoloration by age 50 is common. If you have medium to dark skin and notice a streak or spot that’s been stable for a long time, pigmentation is the likely explanation.
Other benign causes include moles (nevi) within the nail matrix and lentigos, the same type of “age spots” that appear on skin elsewhere on the body.
Fungal and Bacterial Infections
Fungal nail infections can produce dark discoloration, though they more commonly cause yellow, white, or brownish changes. Certain mold species are more likely to create darker pigmentation. The hallmarks of a fungal infection go beyond color: the nail thickens, becomes brittle or crumbly, and may start lifting away from the nail bed. You might also notice debris building up underneath the nail.
Bacterial infections, particularly those caused by Pseudomonas, can turn a nail greenish-black. These infections tend to develop in nails that are already damaged or have separated from the nail bed, creating a moist pocket where bacteria thrive. Without treatment, nail infections generally worsen over time rather than resolving on their own.
Medications and Other Triggers
Certain medications can cause dark streaks or spots in nails as a side effect. Chemotherapy drugs are the most common culprits. Antiretroviral medications used in HIV treatment, antimalarial drugs, and even some antifungal medications can trigger nail pigmentation. If you started a new medication and noticed nail changes afterward, the timing is worth mentioning to your prescriber.
Less common triggers include vitamin B12 deficiency, thyroid conditions, Addison’s disease, psoriasis, and pregnancy. External staining from substances like tobacco, henna, or certain dyes can also darken nails, though this discoloration sits on the surface rather than underneath.
When a Black Spot Could Be Melanoma
Subungual melanoma is rare, but it’s the reason a black toenail spot deserves attention rather than dismissal. It typically appears as a dark longitudinal streak running from the base of the nail toward the tip, rather than as a round bruise-like spot. Dermatologists use an ABCDEF framework to assess suspicious nail streaks:
- Age: adults, particularly between 20 and 90, with higher risk in people of Asian or African descent
- Band: a pigmented streak wider than 3 millimeters that’s brown to black
- Change: the streak is new, widening, darkening, or otherwise evolving
- Digit: most commonly appears on a single nail, often the thumb or big toe
- Extension: pigment spreading beyond the nail onto the surrounding skin (called Hutchinson’s sign)
- Family history: personal or family history of melanoma
Hutchinson’s sign, where dark pigment bleeds onto the skin around the nail, is one of the most important warning signs. In one study of early-stage nail melanoma cases, nearly 90% showed this sign. Any pigment spreading beyond the borders of the nail itself warrants prompt evaluation.
How Doctors Tell the Difference
A dermatologist can often distinguish between a bruise, benign pigmentation, and something more concerning through a combination of history and visual examination, sometimes using a dermoscope (a specialized magnifying tool with polarized light). One simple distinguishing feature: a bruise advances toward the tip of the nail as it grows out over weeks and months. If a dark spot stays fixed in place or keeps expanding, that suggests it’s being produced by something in the nail matrix rather than trapped blood.
When the diagnosis is uncertain, a nail biopsy may be necessary. This involves removing a small sample of the nail and the tissue underneath for microscopic examination. The American Academy of Dermatology recommends seeing a dermatologist for any new or changing dark streak on a nail, any nail that’s lifting from its bed, or any greenish-black discoloration suggesting infection.
What to Watch For
Most black spots on toenails are harmless and resolve on their own as the nail grows out. The spots that deserve a closer look share a few features: they don’t move forward with nail growth, they widen or darken over time, they affect only one nail, or pigment extends onto the skin surrounding the nail. A streak that appeared without any injury you can recall also warrants attention, particularly if you’re over 50 or have a personal or family history of melanoma.