A brown line on your nail is caused by melanin deposits in the nail plate, and whether it goes away depends entirely on what triggered it. In most cases, the line is harmless and will either grow out on its own or resolve once the underlying cause is treated. Fingernails grow about 3.5 mm per month and toenails about 1.6 mm per month, so a line near the base of a fingernail can take four to six months to fully disappear.
What Causes a Brown Line on Your Nail
Your nail plate is made of keratin, which has no color. But pigment-producing cells exist in the nail, and certain triggers can activate them, depositing melanin into the nail as it grows. This creates a visible brown or dark streak running lengthwise from the base of the nail toward the tip.
The most common causes are all noncancerous:
- Nail trauma: Biting, picking, or friction from tight shoes can activate pigment cells. A bruised nail from an injury can also leave a dark streak.
- Moles: A small mole in the nail matrix (the tissue under the base of your nail) can produce a steady brown band.
- Skin tone: People with medium to dark skin tones naturally have more active melanin-producing cells and often develop brown lines without any underlying problem.
- Age spots: Just like on your skin, lentigos can form in the nail.
Less common causes include fungal or bacterial infections, vitamin B12 deficiency, thyroid conditions, Addison’s disease, pregnancy, inflammatory skin conditions like psoriasis, and certain medications. HIV and connective tissue diseases like lupus can also trigger it.
Splinter Hemorrhages Look Similar
Not every dark line is a pigment issue. Tiny blood vessels under the nail can burst, leaking blood that shows through as thin streaks 1 to 3 mm long running in the direction of nail growth. These splinter hemorrhages start out reddish or purple and darken to brown or black within a few days. They usually appear closer to the tip of the nail and grow out as the nail grows. If you can trace the line back to a specific injury or remember banging your finger, a splinter hemorrhage is the likely explanation, and it needs no treatment.
When a Brown Line Is Worth Investigating
Most brown nail lines are benign, but a type of skin cancer called subungual melanoma can look like one. Dermatologists use a set of warning signs to distinguish the two. Features that raise concern include: the line appearing for the first time in adulthood (especially between ages 40 and 70), affecting only a single nail, a band wider than 3 mm, irregular or blurred borders, uneven color within the band, and rapid changes in size or appearance.
The most telling red flag is called the Hutchinson sign, where the dark pigment extends beyond the nail onto the surrounding skin fold. Benign lines tend to have uniform, parallel pigment bands on a consistent background. Malignant lines show irregular streaks that differ in thickness, color, and spacing.
People with darker skin tones face a higher risk of subungual melanoma compared to white populations, with studies showing a 3.4 to 4.5 times increased risk. This is especially important because brown lines are also more common in darker skin for completely benign reasons, making it harder to spot a problem without professional evaluation.
Treating the Underlying Cause
Trauma-Related Lines
If the line came from repeated nail biting, picking, or tight footwear, stopping the source of trauma is the only treatment needed. Once the irritation ends, the pigment cells deactivate and the existing brown line simply grows out with the nail. For fingernails, expect this to take roughly four to six months. Toenails grow about half as fast, so a line on a big toe could take a year or more to fully disappear.
Medication-Related Lines
Certain drugs are known to trigger brown or black bands on nails. Chemotherapy drugs like hydroxyurea, antimalarials like hydroxychloroquine, antiretroviral medications used for HIV, and psoralen (used in light therapy for skin conditions) are the most common culprits. These lines typically fade after the medication is stopped or changed, though it takes a full nail growth cycle to clear. If you suspect a medication is the cause, talk to your prescriber about alternatives rather than stopping on your own.
Fungal Infections
When a fungal infection causes nail discoloration, the line won’t resolve until the infection is treated. Oral antifungal medications are the most effective option and are typically taken for several weeks to months. Topical antifungals applied directly to the nail can help but work best when combined with oral treatment. Fungal nail infections are stubborn, and even after treatment, you’ll need to wait for the damaged nail to grow out completely before it looks normal again.
Nutritional Deficiencies
A vitamin B12 deficiency can activate pigment cells in the nail. If blood work confirms low B12, supplementation or dietary changes (more meat, fish, eggs, or fortified foods) can correct the deficiency. The line will grow out over the following months as healthy nail replaces the pigmented section.
Hormonal and Systemic Conditions
Conditions like Addison’s disease, Cushing’s syndrome, or an overactive thyroid can cause melanin deposits in the nails. Treating the hormonal imbalance typically stops new pigment from forming, and the existing line grows out. Pregnancy-related lines usually resolve after delivery on their own timeline.
What Happens if a Biopsy Is Needed
If your dermatologist sees warning signs, they may recommend a nail matrix biopsy to check for melanoma. The procedure is done under local anesthesia. Your doctor numbs the finger or toe with an injection at the base, then carefully lifts the nail plate to access the tissue underneath where the pigment originates. A thin sample of the nail matrix is removed and sent to a lab.
Stitches are placed to hold everything back together, and they come out about a week later. After that, the nail plate can feel loose or “pop” for a while and may need to be taped down for support. Full nail regrowth takes several months for fingers and longer for toes. The procedure can leave a slight permanent ridge or thinning in the nail, but for ruling out melanoma, the tradeoff is worthwhile.
Practical Steps You Can Take Now
Start by looking closely at the line. If it’s on multiple nails, narrow and uniform, and you have a darker skin tone, it’s very likely benign and may not need any treatment at all. If it appeared after an injury or you’ve been wearing tight shoes, give the nail time to grow out while removing the source of trauma.
Take a photo of the line with a ruler or coin next to it for scale. Check it again in a month. A line that stays the same width, color, and shape over time is reassuring. One that’s getting wider, darker, more irregular, or spreading to the skin around the nail needs a dermatologist’s evaluation sooner rather than later. A single new line on one nail in someone over 40, especially if the band is wider than 3 mm, deserves professional attention even if it hasn’t changed noticeably yet.