What Causes a Black Line Under a Fingernail?

Noticing a black line under your fingernail can be unsettling. In many cases, it’s a harmless sign of minor injury or normal pigment variation. However, sometimes it signals an underlying health issue, including rare but serious conditions like subungual melanoma (a type of skin cancer under the nail). Understanding the possible causes, when to seek care and what to expect during evaluation can help you make informed decisions without undue worry.

Common Causes

1. Trauma or Subungual Hematoma

   • A blow or pinch to the fingertip can rupture tiny blood vessels under the nail, creating a dark red, brown or black line or area.
   • Pain or throbbing is common right after the injury.
   • As the nail grows, the discoloration should gradually move forward and be trimmed away.

2. Melanonychia (Melanin Deposition)

   • Caused by increased pigment (melanin) in the nail matrix.
   • Appears as a brown or black vertical stripe.
   • Can affect one or multiple nails.
   • Common in people with darker skin tones and may be benign.

3. Subungual Melanoma

   • A rare but serious cause of a persistent dark stripe under a nail.
   • Warning signs include:
     • Stripe wider than 3 mm or growing in width
     • Irregular or blurred borders
     • Color variation (brown, black, grey)
     • Pigment that extends onto the adjacent skin (Hutchinson’s sign)
   • Early detection is critical for treatment success.

4. Splinter Hemorrhages

   • Tiny, linear brown or red streaks under the nail that resemble wood splinters.
   • Often due to small blood clots from trauma.
   • Can also be a sign of systemic conditions such as endocarditis (heart valve infection) or autoimmune disease.

5. Fungal or Bacterial Infection

   • Some infections can cause dark discoloration as the organism grows under or within the nail.
   • Often accompanied by nail thickening, crumbling edges, foul odor or surrounding skin redness.

6. Medications and Systemic Conditions

   • Chemotherapy drugs, antimalarials, tetracyclines and some heavy metals may cause melanonychia.
   • Addison’s disease, HIV, malnutrition or vitamin deficiencies can sometimes affect nail pigmentation.

7. Foreign Bodies

   • Tiny particles of dirt, ink or other substances can lodge under the nail and appear as a dark line.
   • Usually removable by gentle cleaning but may require professional trimming if embedded.

When to Seek Medical Advice

While many causes of a black line under fingernail are harmless, certain features warrant prompt evaluation:

• Stripe appears suddenly without clear injury
• Line is wider than 3 mm or rapidly growing in width
• Color is very dark, variegated or black
• Borders are irregular or blurred
• Pigment extends onto the skin around the nail
• Nail changes accompanied by pain, swelling or discharge
• More than one nail affected without a clear cause (like trauma)

If you meet any of these criteria or if you remain concerned, speak to a doctor. You may also consider a free, online symptom check, using the doctor approved Ubie Symptom Checker to get personalized guidance before your appointment.

How It’s Diagnosed

1. Clinical Examination

   • Your healthcare provider will carefully inspect your nail, look for asymmetry, irregular borders and color variation.
   • They will ask about duration, any history of injury and other health changes.

2. Dermoscopy

   • A special magnifying tool helps visualize pigment patterns under the nail to distinguish benign from suspicious streaks.

3. Nail Matrix Biopsy

   • If melanoma is suspected, a small sample of the nail matrix (the tissue that makes the nail) is removed under local anesthesia for laboratory analysis.

4. Additional Tests

   • Blood tests or imaging studies may be ordered if an infection or systemic disease is suspected.

Treatment Options

• Subungual Hematoma
  • Often resolves on its own as the nail grows out (several weeks to months).
  • For pressure relief, a doctor may drill a small hole in the nail to drain trapped blood (trephination).

• Benign Melanonychia
  • No treatment is needed if a benign cause (like a nevus) has been confirmed.
  • Monitor for any changes in size, shape or color.

• Subungual Melanoma
  • Surgical removal of the affected nail and underlying matrix is standard.
  • Additional treatment (chemotherapy, immunotherapy or targeted therapy) may be required depending on staging.

• Infections
  • Topical or oral antifungal or antibiotic medications, depending on the organism.
  • Nail debridement (trimming away infected tissue) in some cases.

• Underlying Conditions
  • Adjust or switch medications if drug-related pigmentation is suspected.
  • Treat systemic diseases (e.g., Addison’s disease, endocarditis) in coordination with specialists.

Tips for Nail Health and Prevention

• Wear protective gloves when doing activities that risk finger injury (gardening, sports, heavy lifting).
• Keep nails trimmed and clean to spot changes early.
• Avoid sharing nail tools or polish to reduce infection risk.
• Moisturize nails and cuticles to prevent splitting or trauma.
• Schedule routine skin exams with a dermatologist if you have a history of skin cancer or multiple pigmented lesions.

Key Takeaways

• A black line under a fingernail may come from simple trauma or more serious causes like subungual melanoma.
• Most benign streaks heal as the nail grows out—medical evaluation isn’t always required.
• Warning signs include a wide, irregular, dark stripe, especially if it expands over time or reaches the skin.
• Early detection of melanoma under the nail greatly improves outcomes.
• When in doubt, speak to a doctor and consider a free, online symptom check, using the doctor approved Ubie Symptom Checker.

If you notice any worrying changes or have symptoms that could be life threatening or serious, do not delay—speak to a medical professional as soon as possible. Your health and peace of mind are worth the check.