What causes brown nail discoloration?
Brown nail discoloration can happen when pigment, blood, infection, or debris changes the nail’s normal color.
In many cases, the cause is harmless and temporary, but some changes can signal an infection, a medication reaction, or an underlying health condition.
The key is to look at the pattern: one nail or several, a streak or a patch, thickening or splitting, and whether the discoloration is growing out with the nail.
Those details often point to the cause.
Common causes of brown nail discoloration
Several everyday and medical factors can turn a nail brown.
The most common causes include trauma, fungal infection, staining, and pigment changes.
1. Trauma or bleeding under the nail
A hit to the finger or toe can cause blood to collect under the nail, leading to brown, red-brown, or black discoloration.
This is called a subungual hematoma.
It often follows dropping something on the foot, stubbing a toe, or repeated pressure from tight shoes.
- Usually affects one nail
- May be painful, tender, or throbbing at first
- Discoloration often moves outward as the nail grows
2. Fungal nail infection
Onychomycosis, or fungal nail infection, is a frequent cause of brown nails, especially in toenails.
The nail may turn yellow-brown, become thickened, crumble at the edges, or separate from the nail bed.
Fungal infections are more likely in warm, moist environments and can spread through communal showers, locker rooms, or shared tools.
They often develop slowly and may affect more than one nail.
3. Nail staining from external substances
Nails can pick up brown stains from things they contact regularly.
Tobacco, hair dye, self-tanners, henna, certain cosmetics, and some chemicals can discolor the nail surface without affecting the nail itself.
Staining is more likely when the nail plate looks otherwise normal and the color is mostly on the surface.
The discoloration may improve as the nail grows out or after gentle cleaning, depending on the substance.
4. Melanonychia
Melanonychia occurs when pigment called melanin is deposited in the nail plate.
It can appear as a brown or black band running lengthwise on the nail.
This can happen because of normal pigmentation in darker skin tones, friction, trauma, infection, certain medications, or less commonly a growth in the nail matrix.
When a dark band appears in a single nail, changes width, or becomes irregular, it deserves medical evaluation because it can occasionally be a sign of subungual melanoma.
5. Medication side effects
Some medications can cause brown or dark nail changes by affecting pigment production or causing bleeding under the nail.
Drugs reported to affect nails include certain chemotherapy agents, antimalarials, tetracyclines, minocycline, and some antiretrovirals.
If discoloration starts after a new medication is introduced, the timing may help connect the change.
Do not stop a prescription on your own; ask the prescribing clinician whether the nail change is expected or concerning.
6. Systemic health conditions
Less commonly, brown nail discoloration can be linked to broader health issues such as Addison’s disease, thyroid disease, vitamin deficiencies, or inflammatory conditions.
These usually cause additional symptoms beyond nail color changes.
Examples include fatigue, skin changes, weight changes, or multiple nails with abnormal growth or color.
Because nail findings alone are rarely enough for a diagnosis, associated symptoms matter.
How to tell whether the cause is likely harmless
Some nail discoloration patterns are more likely to be benign.
A spot that follows a known injury, a stain on the surface, or a fungal change that develops slowly over time is often less urgent than a rapidly changing pigmented band.
More reassuring signs include:
- Color change that is clearly growing out with the nail
- A known cause, such as trauma or staining
- Stable appearance over time
- Involvement of several nails in a pattern consistent with a known condition
Warning signs that need medical evaluation
See a dermatologist, podiatrist, or primary care clinician if the discoloration is new and unexplained, especially if it affects only one nail.
Pigmented nail changes can sometimes be difficult to assess without a professional exam.
Red flags include:
- A single brown or black streak that widens or changes shape
- Irregular borders or uneven color within the nail
- Color extending onto the surrounding skin
- Nail splitting, lifting, bleeding, or persistent pain
- Discoloration that does not grow out
- Darkening without a clear injury or stain
A changing dark band deserves prompt attention because nail melanoma, although uncommon, is easiest to treat when found early.
How doctors diagnose brown nail discoloration
Evaluation usually starts with a visual exam and a history of recent injuries, shoes, hobbies, nail products, and medications.
Clinicians may ask when the discoloration began, whether it is painful, and whether it has changed.
Depending on the appearance, they may use a dermatoscope, trim the nail for fungal testing, or order a biopsy if a pigmented lesion looks suspicious.
The goal is to distinguish harmless pigment changes from conditions that need treatment.
Can brown nail discoloration go away?
In many cases, yes.
Staining, minor trauma, and some temporary pigment changes improve as the nail grows out.
Fingernails usually grow faster than toenails, so toenail discoloration can take much longer to clear.
If the cause is a fungal infection, treatment may include prescription topical or oral antifungal medication.
If the cause is medication-related or linked to an underlying condition, improvement depends on addressing the trigger.
What you can do at home
While waiting for evaluation or watching a mild change, basic nail care can reduce additional damage and make changes easier to monitor.
- Keep nails clean and dry
- Trim nails straight across and avoid picking or scraping
- Wear properly fitted shoes to reduce repeated trauma
- Avoid covering the nail with polish if you need to monitor color changes
- Use gloves for cleaning chemicals or staining substances
If you suspect a fungal infection, avoid sharing nail tools and footwear, and disinfect clippers regularly to reduce spread.
Which nail changes are most likely to be fungal?
Fungal nail infections often cause more than color change.
Look for thickening, brittle texture, crumbly edges, lifting from the nail bed, and debris under the nail.
Toenails are affected more often than fingernails, especially in people with athlete’s foot, diabetes, reduced circulation, or frequent exposure to moisture.
Because fungal infections can resemble trauma or psoriasis, testing is useful before starting treatment.
A confirmed diagnosis helps avoid unnecessary medication and improves treatment accuracy.
Why location and pattern matter
Where the discoloration appears can help narrow the cause.
A bruise under the nail usually starts near the point of impact, while staining often affects the surface.
Melanonychia commonly appears as a longitudinal band, and fungal changes often begin at the free edge or sides of the nail.
Pattern clues that matter include:
- One nail versus multiple nails
- Band-shaped versus patchy discoloration
- Surface stain versus pigment within the nail
- Associated thickening, lifting, or pain
These details do not replace a diagnosis, but they can help a clinician decide which tests are most appropriate.
When brown nail discoloration should be taken seriously
Brown nail discoloration should be taken seriously when it appears without explanation, changes quickly, affects only one nail in a suspicious way, or comes with pain, bleeding, or skin pigment around the nail.
The same applies when the nail looks distorted, thickened, or partially detached and the cause is not obvious.
Because nail changes are visible and measurable over time, taking photos every couple of weeks can help track whether a spot is growing out, staying the same, or evolving in a concerning way.