Black Toenails: Causes, Treatments, and When to See a Doctor

Noticing a black toenail can be startling, but in most cases, it's nothing to worry about. The discoloration is usually the result of a simple bruise under the nail from an injury. However, it's important to understand the different potential causes, as a black toenail can sometimes signal a fungal infection or a more serious underlying health condition that requires medical attention.

This comprehensive guide will walk you through the common and uncommon causes of black toenails, how to tell them apart, available treatments, and crucial prevention tips.

Common Causes of Black Toenails

The discoloration you see is often blood trapped under the nail plate, a condition known as a subungual hematoma. However, other factors can also lead to a blackened nail.

Trauma and Injury (Subungual Hematoma)

This is by far the most frequent cause of a black toenail. Trauma can happen in two ways:

• Acute Injury: A single, direct impact, such as dropping a heavy object on your foot or stubbing your toe hard. This can break the small blood vessels under the nail, causing blood to pool and create a dark, painful spot.
• Repetitive Trauma: This occurs from repeated, minor impacts over time. It's common in athletes, especially runners, and is often called "runner's toe." The constant friction of the toe hitting the inside of a shoe—particularly if the footwear is too tight or too loose—can lead to bruising under the nail.

A black toenail from trauma will typically grow out as the nail itself grows, a process that can take several months.

Fungal Infections (Onychomycosis)

Fungal infections can also cause toenails to change color. While they often appear yellow or brown, an accumulation of debris under the nail can give it a dark or black appearance. Fungi thrive in warm, moist environments, making sweaty shoes and socks a common breeding ground. Besides discoloration, a fungal infection may cause the nail to become:

• Thickened
• Brittle or crumbly
• Distorted in shape
• Accompanied by a foul odor

Underlying Health Conditions

Sometimes, a black toenail can be a window into your overall health. Certain systemic conditions can affect blood flow and nail health, leading to discoloration. These include:

• Diabetes: Poor circulation and nerve damage (neuropathy) associated with diabetes can make feet more vulnerable to injury and infection.
• Kidney or Heart Disease: These conditions can impact circulation, affecting the health of your extremities.
• Anemia: A lack of healthy red blood cells can sometimes manifest in nail changes.
• Autoimmune Diseases: Conditions like psoriasis can affect the nail plate and bed, causing various changes including discoloration.

Nutritional Deficiencies

While less common, a lack of certain vitamins and minerals can impact nail health. A Vitamin B12 deficiency is most notably linked to hyperpigmentation, which can cause blue-black discoloration of the nails.

Medication Side Effects

Certain medications are known to cause nail changes as a side effect. These can include some chemotherapy drugs, antibiotics (like tetracyclines), and retinoids.

A Rare But Serious Cause: Melanoma

In rare instances, a black or dark brown streak under the nail can be a sign of subungual melanoma, a serious form of skin cancer. Unlike a bruise that grows out with the nail, a melanoma streak typically remains in place or expands over time. Key warning signs include:

• A dark streak that appears without an injury.
• The streak widens, especially near the cuticle.
• The dark pigment spreads to the surrounding skin (known as Hutchinson's sign).
• The nail begins to split, bleed, or lift from the nail bed.

When to See a Doctor

While most black toenails from an injury will resolve on their own, it's crucial to seek medical advice from a podiatrist or dermatologist if you notice any of the following:

• You can't recall any injury that would have caused the discoloration.
• The black area is growing or changing shape.
• The discoloration has not started to grow out with the nail after a few weeks.
• You are experiencing severe pain, swelling, redness, or discharge, which could indicate an infection.
• The dark color spreads to the skin around the nail.
• You have an underlying condition like diabetes, peripheral artery disease, or an autoimmune disorder.

Treatment and Management for Black Toenails

Treatment depends entirely on the underlying cause. A proper diagnosis from a healthcare professional is the first and most important step.

Treating Trauma-Induced Black Toenails

• Mild Cases: If the bruise is small and the pain is minimal, no treatment is needed. The discolored area will simply grow out as your nail grows.
• Severe Pain: If the pressure from the pooled blood is causing significant pain, a doctor can perform a procedure called nail trephination. They will create a tiny hole in the nail to drain the blood and relieve the pressure. This should never be attempted at home.
• If the Nail Falls Off: Sometimes, a severe injury can cause the nail to separate from the nail bed and fall off. If this happens, keep the area clean and protected with an antibiotic ointment and a bandage until a new nail grows in, which can take over a year.

Treating Fungal Infections

Fungal infections can be persistent and require consistent treatment.

• Home Remedies: For mild cases, some people try natural antifungal agents like tea tree oil, oregano oil, or vinegar soaks. However, scientific evidence for their efficacy is limited, and they can take a long time to work.
• Medical Treatments: A doctor can recommend more effective treatments, including:
  • Topical Antifungals: Prescription-strength creams, polishes, or liquids applied directly to the nail.
  • Oral Antifungals: Pills like terbinafine or itraconazole are very effective but may have side effects and require monitoring.
  • Laser Therapy: A newer option that uses light to destroy the fungus.

Healing Timelines: Patience is Key

Regardless of the cause, healing a toenail is a slow process. A toenail only grows about 1.6 mm per month. It can take 12 to 18 months for a damaged nail to be fully replaced by a new, healthy one.

Prevention Strategies

You can significantly reduce your risk of developing a black toenail with a few simple habits.

For Everyone

• Wear Properly Fitting Shoes: Ensure there is about a thumb's width of space between your longest toe and the end of your shoe. This prevents your toes from jamming into the shoe.
• Keep Nails Trimmed: Cut your toenails straight across and keep them relatively short to prevent them from hitting the front of your shoes.
• Maintain Good Foot Hygiene: Keep your feet clean and dry to create an inhospitable environment for fungus. Wear moisture-wicking socks and avoid walking barefoot in public, damp areas like pools or gym locker rooms.

Special Considerations for High-Risk Groups

• Athletes: In addition to proper footwear, consider using different lacing techniques to secure your heel and prevent your foot from sliding forward. Some athletes, like Cristiano Ronaldo, have even been known to paint their toenails with black polish, a trend from MMA fighters thought to harden the nail and protect against fungal infections.
• People with Diabetes: Daily foot inspection is critical. Check for any cuts, blisters, or color changes. Always wear comfortable, well-fitting shoes and see a podiatrist regularly for foot exams.

Frequently Asked Questions (FAQ)

What does it mean if a toenail turns black? A black toenail is most often a bruise under the nail (subungual hematoma) caused by an injury. However, it can also signal a fungal infection, an underlying medical condition like diabetes, or in rare cases, a serious form of skin cancer called melanoma.

What vitamin deficiency causes dark toenails? A Vitamin B12 deficiency is most commonly associated with dark or black toenails. This deficiency can cause hyperpigmentation, leading to blue-black discoloration on the nails.

How do you get rid of black toenails naturally? If the black toenail is from a minor injury, it will typically grow out on its own. For black toenails caused by fungus, some natural remedies include applying tea tree oil or oregano oil. Maintaining good foot hygiene is also crucial.

Does a black toenail mean you have diabetes? Not necessarily, but it can be a sign of complications associated with diabetes, such as poor circulation or nerve damage. If you have diabetes and develop a black toenail, see your doctor promptly.

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References

1. Fletcher, J. (2024). 6 potential causes of a black toenail. Medical News Today. https://www.medicalnewstoday.com/articles/320187
2. LeBrun, N. (2025). 6 Causes of a Black Toenail and How to Treat It. Verywell Health. https://www.verywellhealth.com/black-toenail-7197905
3. Foot & Ankle Surgical Associates. (2024). Causes & Treatments for Black Toenails. https://fasafw.com/blog/causes-and-treatments-for-black-toenails/
4. American Academy of Dermatology. Nail melanoma. https://www.aad.org/public/diseases/skin-cancer/types/common/melanoma/nail-melanoma