A black or dark brown vertical line on your toenail is usually a deposit of melanin pigment in the nail plate, a condition called longitudinal melanonychia. In most cases it’s completely harmless, but it can occasionally signal something serious, including melanoma. The line typically runs from the base of the nail to the tip and may appear on one or several nails.
What the line means depends on its appearance, how it changes over time, and your individual risk factors. Here’s how to tell the difference between the common, benign causes and the rare but important warning signs.
Normal Pigmentation Is the Most Common Cause
The single most frequent reason for a dark line on a toenail is simply your body producing melanin in the nail matrix, the tissue at the base of the nail where new nail cells form. This is especially common in people with darker skin tones. Over 77% of Black adults older than 20 have pigmented nail bands, and about 20% of people of Japanese descent do as well. In white populations the prevalence is around 1.4%.
These lines are a normal variant, not a disease. They tend to appear gradually, may show up on multiple nails, and stay relatively stable in width and color over time. If you have darker skin and notice evenly colored, thin streaks that haven’t changed much, there’s a good chance this is the explanation.
Trauma and Bruising Under the Nail
Stubbing your toe, dropping something on your foot, or wearing tight shoes can cause bleeding under the nail. This pooled blood (a subungual hematoma) often looks like a dark reddish-brown or black spot or streak. The key difference between a bruise and a pigmented line is that a bruise moves forward as your nail grows. Toenails grow slowly, about 1.6 mm per month, so it can take many months for a bruise to reach the tip and grow out entirely.
A bruise also tends to change color over time, shifting from dark purple-black to brownish-red as the blood breaks down. If you remember a specific injury and the dark area is slowly migrating toward the end of your nail, you’re almost certainly looking at trapped blood rather than pigment.
Fungal Infections
Certain fungi produce dark pigment that can stain the nail plate black or brown. This isn’t the typical yellowed, thickened toenail most people picture when they think of nail fungus. Pigment-producing fungi include species that naturally contain melanin in their cell walls, but even common nail fungi can occasionally cause dark discoloration. A systematic review identified more than a dozen fungal species capable of producing melanonychia.
Fungal melanonychia may affect one or more nails and is often accompanied by other signs of infection: thickening, crumbling, or lifting of the nail plate. If the line appeared alongside these textural changes, a fungal cause is worth investigating. A nail clipping or scraping can confirm the diagnosis, and treatment with antifungal medication typically resolves the discoloration as healthy nail grows in.
Medications That Darken Nails
Several classes of drugs can trigger dark streaks or overall nail discoloration as a side effect. Chemotherapy drugs are among the most well-known causes. Cisplatin, for example, produces hyperpigmentation in roughly 70% of patients, often affecting the nails, hair, and mouth. Other cancer drugs, certain HIV medications (particularly zidovudine), antimalarials, some antiseizure medications like valproic acid, and even long-term use of certain psychiatric medications can darken the nails.
Drug-related nail changes usually affect multiple nails and appear weeks to months after starting the medication. The discoloration often fades after the drug is stopped, though it takes time because the new, unpigmented nail has to grow in from the base.
When a Black Line Could Be Melanoma
Subungual melanoma, a type of skin cancer that starts in the nail matrix, is rare but serious. It accounts for a significant proportion of melanomas in people with darker skin, making up about 50% of melanomas diagnosed in dark-skinned populations. Early detection matters because this cancer can spread to other parts of the body.
Dermatologists use a set of features to distinguish suspicious lines from benign ones:
- Width of 3 mm or more, especially if the band is widening over time
- Irregular borders with uneven edges rather than crisp, parallel lines
- Color variation within the band, mixing shades of brown, black, and gray rather than one uniform tone
- Pigment spreading beyond the nail onto the surrounding skin of the cuticle or nail fold, known as the Hutchinson sign
- A single nail affected, particularly on the thumb or big toe
The Hutchinson sign, where dark pigment bleeds into the skin around the nail, is one of the strongest visual clues pointing toward melanoma. If you notice any of these features, a dermatologist can evaluate the nail and determine whether a biopsy is needed.
How Doctors Diagnose the Cause
A dermatologist will typically start by examining the nail with a dermatoscope, a handheld magnifying device with a built-in light. Under magnification, benign pigmentation tends to show regular, evenly spaced thin lines, while melanoma produces irregular lines with varying thickness and spacing against an uneven background.
If the pattern looks suspicious, the next step is a nail matrix biopsy. This involves removing a small sample of tissue from the base of the nail where the pigment originates. Several techniques exist, including punch biopsy and a method sometimes called the “trap door” approach, which lifts a portion of the nail plate to give the doctor a direct view of the matrix so the biopsy can target the exact area producing the pigment. The procedure is done under local anesthesia. Some temporary nail irregularity during regrowth is normal, and doctors aim to minimize this by biopsying the part of the matrix least likely to cause lasting changes to the nail surface.
Tracking Changes at Home
Because toenails grow so slowly (roughly 1.6 mm per month, meaning a full toenail can take over a year to replace itself), changes happen gradually and can be hard to notice in real time. Taking a close-up photo of the nail every month or two gives you a reliable way to spot shifts in width, color, or border regularity. Place a small ruler or coin next to the nail for scale.
A line that stays the same width, color, and pattern over several months is reassuring. A line that is new, widening, darkening, developing multiple colors, or spreading pigment into the skin around the nail deserves prompt evaluation. The same is true for a dark area that doesn’t grow out with the nail the way a bruise would. Even if the odds favor a benign explanation, the stakes of missing a melanoma are high enough that getting a professional look is worthwhile.