You're washing your hands, or maybe just glancing down while typing, and you see it. A dark, thin streak running vertically down your fingernail. It looks like someone took a fine-tip Sharpie and drew a single line from the cuticle to the tip. Your heart probably sank a little bit because the internet has a way of telling everyone that every minor physical quirk is a terminal illness. Take a breath. While you definitely shouldn't ignore it, those black lines in my nails usually have a much more boring explanation than the worst-case scenario.
Most of the time, these are what doctors call splinter hemorrhages. They’re basically tiny blood clots that have migrated up the nail. But sometimes, it's something called a melanonychia. That's a fancy word for pigment in the nail. We need to talk about why these show up, how to tell the difference between a bruise and something more serious, and what you should actually say to a dermatologist.
What are the black lines in my nails, exactly?
If the line is thin, reddish-brown, or black and looks like a literal splinter under the skin, it’s almost certainly a splinter hemorrhage. This happens when the tiny capillaries under your nail bed get damaged and leak a tiny amount of blood. The blood gets trapped. As the nail grows out, it carries that little bit of dried blood with it.
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Why does this happen? Usually, you just hit your hand. You might not even remember doing it. Think about how often we bump our fingers against car doors, keyboards, or kitchen counters. For most people, that’s the end of the story. However, if you have these lines on multiple fingers and you haven't had any recent trauma, doctors like Dr. Shari Lipner from Weill Cornell Medicine often look for underlying systemic issues. Sometimes, it can be a side effect of certain medications like aspirin or even a sign of a heart valve infection called endocarditis, though that usually comes with a high fever and feeling like absolute garbage.
The pigment factor
Now, if the line is a consistent band of brown or black that doesn't look like a "splinter," you’re likely looking at melanonychia. This is where your melanocytes—the cells that produce melanin—start depositing pigment into the nail as it forms.
This is incredibly common in people with darker skin tones. In fact, by age 50, a huge percentage of Black individuals will have at least one of these lines. It’s often just a natural variation, like a freckle but for your nail bed. It's called longitudinal melanonychia. It’s usually benign. But, and this is the part where we have to be serious, it can also be a sign of subungual melanoma. That is a rare but dangerous form of skin cancer.
The "ABCDEF" rule for nail streaks
How do you know if you need to run to the doctor tomorrow or just wait for your next checkup? Dermatologists use a specific set of criteria to evaluate black lines in my nails. It’s an easy mental checklist you can use at home.
A is for Age. Subungual melanoma is most commonly diagnosed in people between the ages of 50 and 70. It’s quite rare in children.
B is for Band. Look at the width and the borders. Is the line wider than 3 millimeters? Are the edges blurry or jagged instead of a crisp, straight line? A "wide" band is a red flag.
C is for Change. This is the big one. If the line was thin and light brown two months ago, but now it’s wide, dark black, and the shape is changing, that’s a problem. Evolution of the streak is the most significant warning sign.
D is for Digit. Statistics show that melanoma is most likely to appear in the "power digits." We are talking about the thumb, the big toe, or the index finger. If the line is on your pinky, it’s statistically less likely to be malignant, though not impossible.
E is for Extension. Take a close look at the skin around the nail, specifically the cuticle. If the pigment is "bleeding" onto the skin of the cuticle or the nail fold, this is known as Hutchinson’s sign. This is a very strong indicator that you need a biopsy immediately.
F is for Family history. If your family has a history of melanoma, your threshold for getting things checked out should be much lower.
Common culprits that aren't cancer
It’s easy to spiral into a Google-induced panic. Let’s look at the "not scary" stuff.
Fungal infections can sometimes cause dark discolorations. It usually looks more like a patch than a clean line, but it can be confusing. Certain medications, especially chemotherapy drugs or even some anti-malarial meds, can cause your nails to develop dark stripes. Even pregnancy can trigger increased pigment production in the nails.
Then there’s the "friction" factor. If you wear shoes that are too tight, the constant rubbing on your toes can cause "activational" melanonychia. Your body reacts to the trauma by turning on the pigment cells. If you see these lines on your toes, check your footwear. Are you a runner? Subungual hematomas (big bruises under the nail) are a rite of passage for long-distance runners, often resulting in a nail that turns completely black and eventually falls off.
When it’s just a bruise
A bruise (hematoma) will grow out with the nail. If you take a photo of your nail today and another one in three weeks, a bruise will have moved closer to the tip. A melanonychia, because it's coming from the "root" (the matrix), will stay anchored at the base and just keep producing a long, continuous stripe.
The biopsy process: What to expect
If you go to a dermatologist and they don't like the look of those black lines in my nails, they’ll suggest a biopsy. I’ve talked to many people who are terrified of this. It sounds painful. Honestly, they numb the finger completely. You won't feel the procedure itself.
The doctor will usually take a "punch biopsy" or a "shave biopsy" of the nail matrix. This is the area under your cuticle where the nail is born. They need to see if the cells there are behaving themselves. The downside? It might leave a permanent split in your nail or a permanent line. But considering the alternative is missing a melanoma diagnosis, it’s a trade-off most people are willing to make.
Moving forward with your nail health
Don't just paint over it. A lot of people see a dark line and think, "I'll just keep a coat of dark polish on until it goes away." That is the worst thing you can do. It prevents you from noticing if the line is changing or widening.
If you have a dark line that appeared out of nowhere and isn't moving toward the tip of your finger as the nail grows, make an appointment. You don't need an emergency room, but you do need a specialist.
Actionable steps for your next steps
- Document the line. Take a clear, high-resolution photo of your nail next to a ruler. Do this today.
- Wait and watch. Check the line again in 4 weeks. If it has moved further away from the cuticle, it’s likely a bruise. If it has stayed put at the base or gotten wider, call the dermatologist.
- Check your shoes. If the lines are on your toes, look for signs of crowding. Switch to a wider toe box for a month and see if new growth is clear.
- Review your meds. Look up the side effects of any new prescriptions you've started in the last six months.
- Remove all polish. Before your doctor's appointment, make sure every scrap of nail polish is gone from all ten fingers and toes. Doctors need to see the entire nail bed and the surrounding skin to make an accurate assessment.
Bottom line: most black lines in my nails are just the result of a clumsy moment or a harmless "nail freckle." But because the exceptions are serious, your eyes are your best defense. If it changes, get it checked.