A brown spot on your nail is almost always harmless. The most common causes are minor injuries, natural pigmentation related to skin tone, and fungal infections. In rare cases, a brown spot or streak can signal something more serious, including melanoma. Knowing what to look for helps you figure out which category yours falls into.
Bruising Under the Nail
The single most common reason for a brown or dark spot on a nail is a bruise underneath it, called a subungual hematoma. You might not even remember the injury. Stubbing a toe, closing a finger in a drawer, or even repetitive pressure from tight shoes or typing can cause a small pool of blood to form between the nail and the nail bed. That trapped blood appears as a dark brown, reddish-brown, or purplish spot.
The key feature of a bruise is that it moves. Fingernails grow at roughly 3.5 millimeters per month, so over the course of weeks you should see the spot gradually shift toward the tip of your nail. A bruise on a fingernail typically grows out completely within two to three months. Toenails are slower and can take up to nine months. If you mark the edge of the spot with a photo or a tiny dot of nail polish, you can track whether it’s migrating forward. A spot that stays in place and doesn’t grow out is worth getting checked.
Natural Pigmentation and Melanonychia
If you have medium to dark skin, brown lines or bands running vertically from the base of the nail to the tip are extremely common and perfectly normal. This is called melanonychia, and it happens when the pigment-producing cells in the nail root deposit melanin into the nail plate as it grows. It can appear on one nail or several, and it often shows up in adulthood without any particular trigger.
Beyond skin tone, other benign causes of melanonychia include moles (nevi) within the nail matrix, age spots forming in the nail, repeated trauma like habitual nail biting or picking, and hormonal changes during pregnancy. These pigmented bands tend to be uniform in color and width, with clean, well-defined edges.
Fungal Nail Infections
A fungal infection often starts as a white or yellowish-brown spot near the tip of the nail. As it progresses deeper, the discoloration can darken to brown, and the nail itself starts to change: thickening, becoming brittle or ragged at the edges, crumbling, or pulling away from the nail bed. You might also notice an unpleasant smell. If your brown spot came with any of these texture changes, a fungal infection is a likely culprit. Toenails are affected more often than fingernails because fungi thrive in warm, moist environments like shoes. Over-the-counter antifungal treatments can work for mild cases, but deeper infections usually need prescription medication.
A greenish or black discoloration, on the other hand, points more toward a bacterial infection rather than a fungal one.
Medications and Medical Conditions
Certain medications can cause brown or dark bands to appear across your nails. Chemotherapy drugs are well-known for this, but antimalarial medications like hydroxychloroquine can also cause a brownish discoloration of the nail bed. If you started a new medication and then noticed nail changes, the timing is probably not a coincidence.
A few systemic health conditions can also show up in your nails. An overactive thyroid can cause brown nail discoloration. In people with advanced kidney disease, nails sometimes develop a distinctive “half-and-half” pattern where the bottom half turns white and the outer half turns brown. Excessive fluoride intake is another uncommon cause. These are relatively rare explanations, but they’re worth knowing about if you have other unexplained symptoms alongside the nail change.
When a Brown Spot Could Be Melanoma
Melanoma can develop under a nail, and it’s one of the reasons a new or changing brown spot deserves attention. Nail melanoma is uncommon overall, accounting for roughly 0.7 to 3.5% of all melanomas in white-skinned populations. However, it represents a much larger share of melanoma cases in people with darker skin tones and Asian populations, where it can account for up to 75% of melanomas. This makes it particularly important not to dismiss pigmented nail changes in these groups.
Dermatologists use a set of criteria sometimes called the ABCDEF rule to evaluate suspicious nail pigment:
- Age: Peak incidence is between the 50s and 70s.
- Band: A brown-to-black band wider than 3 millimeters, with uneven color or irregular borders.
- Change: The band is getting wider, darker, or more irregular over time.
- Digit: The thumb, index finger, and big toe are most commonly involved.
- Extension: Pigment spreading beyond the nail onto the surrounding skin (called Hutchinson’s sign) is a significant warning sign.
- Family history: A personal or family history of melanoma or atypical moles raises risk.
The most important distinguishing feature is behavior over time. A bruise grows out with the nail. A melanoma does not. A melanoma lesion stays anchored in place because it originates from the nail matrix itself, not from a one-time event trapped in the nail plate. If a dark spot has been sitting in the same location for months without moving, that’s a red flag.
Hutchinson’s sign, where dark pigment bleeds from under the nail onto the cuticle or the skin alongside the nail, is another strong indicator. It doesn’t always mean melanoma, but it always warrants a dermatologist visit.
How to Monitor a Brown Spot at Home
If your spot appeared after an obvious injury and you can see it slowly migrating toward the nail tip over weeks, you can feel reasonably confident it’s a bruise. Take a photo of your nail with something for scale (like a coin next to it) and compare it every two to three weeks. A normal bruise will show a clear gap of healthy nail growing in behind it at the base.
You should get a dermatologist to evaluate the spot if any of the following apply: the spot is not growing out with the nail, it’s getting wider or darker, the edges are blurry or irregular, pigment has spread onto the skin around the nail, you have no memory of injuring the nail, or you have a personal or family history of melanoma. A dermatologist can examine the nail with a specialized magnifying tool called a dermoscope, and if there’s any doubt, a small biopsy of the nail matrix can provide a definitive answer.
Most brown nail spots turn out to be completely benign. But because nail melanoma is one of the more commonly delayed cancer diagnoses, largely because people assume it’s just a bruise, it’s worth paying attention to the clues your nail is giving you.