A black nail can be alarming, but the cause is often straightforward. The most common reason is trauma, such as dropping something on your toe or stubbing it, which causes bleeding under the nail. However, black nails can also result from fungal infections, certain medications, or rarely, a serious condition like melanoma. Identifying the underlying cause is the first step toward the right treatment.
What Causes a Black Nail?
The color change you see is usually blood, pigment, or debris trapped under the nail plate. The exact cause determines how it looks, whether it hurts, and how you should treat it.
Most black nails are harmless. But because a rare form of skin cancer can also appear as a dark streak or patch, it is worth understanding what you are looking at.
Trauma: The Most Common Reason Your Nail Is Black
Injury is the leading cause of a black nail. When you hit your finger or toe, small blood vessels under the nail break. The blood pools and hardens beneath the nail plate, creating a dark area called a subungual hematoma.
The color can range from dark red to nearly black. If the injury was recent, the nail may be tender or throbbing. If the injury happened weeks ago, the pain is usually gone, and the dark spot may be growing out with the nail.
This is not dangerous. The trapped blood will grow out as the nail grows. A fingernail typically takes about six months to fully replace itself. A toenail can take up to 12 months or longer.
Fungal Infections Can Cause Black or Dark Nails
Fungal nail infections are common, especially in toenails. Most fungal infections cause yellow, brown, or white discoloration. But some fungi produce dark pigments that make the nail look black or greenish-black.
These infections often start at the edge of the nail and spread toward the cuticle. The nail may become thick, brittle, or crumbly. There may also be a foul odor.
Fungal infections do not resolve on their own. Treatment usually involves prescription antifungal medication taken by mouth for several months. Over-the-counter topical treatments have limited success, especially for toenails.
Medications and Medical Conditions That Darken Nails
Certain drugs can cause nail discoloration as a side effect. Chemotherapy agents are the most well-known culprits. Some antimalarial drugs and certain antibiotics can also darken the nail bed.
This type of discoloration typically affects multiple nails at once. It often appears as dark bands or lines rather than a single solid patch. The color usually fades after the medication is stopped.
Some systemic conditions can also darken nails. Kidney disease, liver disease, and anemia have all been associated with nail color changes. If you have no history of injury and more than one nail is affected, it is reasonable to mention it to your doctor.
When a Black Nail Could Be Melanoma
Melanoma can develop under the nail. This is called subungual melanoma. It is rare, accounting for a small percentage of all melanoma cases, but it is serious.
There are warning signs that distinguish melanoma from a bruise. A dark streak that runs the length of the nail, from cuticle to tip, is a red flag. So is darkening that extends into the skin around the nail. If the pigment grows or changes shape over time, that is also concerning.
A subungual hematoma from trauma usually has a clear history of injury. It also tends to grow out with the nail rather than expand. If you cannot remember injuring the nail, or if the dark area is not moving with the nail as it grows, see a dermatologist.
Melanoma under the nail is often painless in its early stages. Do not wait for pain to seek evaluation.
How to Tell the Difference Between a Bruise and Something Serious
Ask yourself these questions:
- Do you remember injuring the nail?
- Is the dark area confined to the nail plate?
- Has the spot been moving outward as the nail grows?
- Does the pigment extend into the skin around the nail?
- Is there a single dark streak running the full length of the nail?
A bruise from trauma usually checks the first three boxes. Melanoma often does not. If the pigment touches the surrounding skin or forms a streak, a biopsy may be necessary to rule out cancer.
Dermatologists use a tool called a dermatoscope to examine nail pigment more closely. This is a quick, painless exam that can help distinguish a bruise from something more concerning.
Treatment Options for a Black Nail
Treatment depends entirely on the cause.
For a recent injury with throbbing pain, a doctor can drain the trapped blood. This is called trephination. A small hole is made in the nail to release the pressure. This provides immediate pain relief but does not change the appearance. The nail may still fall off weeks later.
If the nail is already falling off, let it separate naturally. Pulling it off can damage the nail bed and lead to an abnormal new nail. Keep the area clean and protected while it heals.
For fungal infections, oral antifungal medication is the most effective option. Treatment lasts several months, and the nail may still look discolored until it fully grows out.
If melanoma is diagnosed, surgery is required. The extent of surgery depends on how deep the melanoma has grown. Early detection makes a significant difference in outcome.
Prevention: Protecting Your Nails
Most black nails from trauma are preventable. Wear shoes that fit properly, especially during sports or hiking. Trim toenails straight across to reduce pressure against the shoe. If you run or hike regularly, keep toenails short to prevent them from jamming into the front of your shoe.
To reduce fungal infection risk, keep feet dry and change socks daily. Wear flip-flops in communal showers or pool areas. Avoid sharing nail clippers or files.
These habits will not prevent every black nail. But they address the two most common causes.
When to See a Doctor
You should see a doctor if any of the following apply:
- The dark area does not grow out with the nail over several weeks
- The pigment extends from the nail into the surrounding skin
- There is a dark streak running the full length of the nail
- You have no history of injury
- Multiple nails are affected at once
- The nail is painful, draining, or has a foul odor
A primary care doctor can evaluate most nail concerns. A dermatologist is the specialist for anything suspicious. If melanoma is a possibility, do not delay the appointment.
Frequently Asked Questions
How long does a black nail take to grow out?
A fingernail takes about six months to fully grow out, while a toenail can take up to 12 months or longer. The dark area will move toward the tip as the new nail grows in.
Can a black nail be a sign of cancer?
Rarely, yes. Subungual melanoma can appear as a dark streak or patch under the nail, but it is uncommon compared to trauma or fungal causes.
Should I drain a black nail at home?
No. Draining a nail at home carries a risk of infection and may damage the nail bed. A doctor can do this safely if the pressure is causing significant pain.
Will my black nail fall off?
Sometimes. If the injury was severe enough to separate the nail from the nail bed, the nail may fall off as a new one grows underneath. This is normal and usually not a cause for concern.
