Why Toenails Turn Black: Causes, Treatment & When to Worry
A black toenail is usually harmless, but not always. Here's how to tell the difference and what to do about it.
A black toenail can be alarming, but in most cases it's not a sign of anything serious. At ASG Foot & Ankle Specialists, we see black toenails regularly, and the vast majority are caused by trauma or fungal infection. That said, there are rare cases where a black toenail signals something that needs prompt attention.
Here's a complete guide to the causes of black toenails, how each is treated, and the warning signs that mean you should see a podiatrist right away.
Medically reviewed by Dr. Timothy Horak and Julia Shauger, board-certified podiatric specialists serving Homewood, South Chicago Heights & Mokena. Last reviewed July 2026.
Common Causes of Black Toenails
Subungual Hematoma (Blood Under the Nail)
The most common cause. Blood pools under the nail after trauma, such as dropping something on your toe, stubbing it, or repetitive pressure from running or tight shoes. The blood appears black or dark red/purple under the nail.
- • Appears immediately or within hours of injury
- • May be painful due to pressure buildup
- • Nail may eventually fall off as blood separates it from nail bed
- • A big toenail takes roughly 12 to 18 months to fully grow back, the smaller toes less
- • Treatment: draining if painful; otherwise leave it alone
Runner's Toe (Repetitive Trauma)
Common in runners, hikers, and athletes. Repeated impact of the toe against the front of the shoe causes micro-trauma and bleeding under the nail. Often affects the longest toe.
- • Gradual onset over weeks or months
- • Often affects the big toe or second toe
- • Caused by shoes that are too small or too large
- • Prevention: properly fitted running shoes with thumb's width of space
- • Treatment: better footwear, nail trimming, padding
Fungal Nail Infection (Onychomycosis)
Advanced fungal infection can cause the nail to turn dark brown or black, in addition to thickening, crumbling, and separating from the nail bed. The discoloration is from debris and fungal pigments accumulating under the nail.
- • Gradual onset over months
- • Nail also thickens, crumbles, or becomes distorted
- • May have foul odor
- • Contagious, and it can spread to other nails and the surrounding skin
- • Treatment: antifungal medications (topical or oral), laser therapy
Subungual Melanoma
A rare but serious cause. Subungual melanoma is a type of skin cancer that develops under the nail. It appears as a dark streak or patch that doesn't move as the nail grows. It can be mistaken for a bruise, which is why any unexplained black toenail should be evaluated.
- • A dark band running lengthwise from the nail base to the tip (longitudinal melanonychia)
- • Pigment spreading onto the cuticle or the skin around the nail (Hutchinson's sign)
- • Doesn't move as nail grows out
- • Irregular borders, multiple colors, or a band that widens over time
- • No history of trauma to explain it
- • Requires biopsy for diagnosis, so see a podiatrist promptly
When to See a Podiatrist Immediately
- • Black area doesn't move as the nail grows out over weeks
- • No injury or trauma to explain the discoloration
- • Dark streak running lengthwise from the nail base to the tip, or pigment spreading onto the cuticle or surrounding skin
- • Irregular borders, multiple colors, or spreading discoloration
- • Signs of infection: pain, swelling, redness, pus, fever
- • You have diabetes, in which case any nail change needs prompt evaluation
- • The nail is extremely painful and you need pressure relieved
Been told it is fungus? OTC Toenail Fungus Treatments Compared →
Dealing with a fungal nail? Laser therapy is one of the in-office options we offer for stubborn fungal nails, alongside topical and oral antifungals. Your podiatrist can tell you which is the better fit after examining the nail. Laser Fungal Nail Therapy →
Frequently Asked Questions
Why is my toenail turning black?
The most common causes of a black toenail are: subungual hematoma (blood pooling under the nail from trauma), fungal nail infection (onychomycosis), repetitive pressure from running or tight shoes, and rarely, subungual melanoma (a type of skin cancer). Most black toenails are caused by trauma or fungal infection and are not dangerous, but any black toenail that appears without a clear cause, doesn't grow out, or has irregular borders should be evaluated by a podiatrist.
Should I be worried about a black toenail?
Most black toenails are caused by trauma (blood under the nail) and resolve on their own as the nail grows out. However, see a podiatrist if: the black area doesn't move as the nail grows, there was no injury to explain it, the discoloration has irregular borders or multiple colors, you have a streak of dark pigment running from the nail base to the tip, or you have diabetes or a weakened immune system.
Will a black toenail fall off?
A black toenail from trauma (subungual hematoma) often does fall off as the blood separates the nail from the nail bed. This is normal, and a new nail will grow in its place. Toenails grow slowly, so a big toenail usually takes about 12 to 18 months to fully replace itself, and the smaller toes are quicker. Keep the area clean and protected. If the nail is painful, a podiatrist can drain the blood to relieve pressure.
See a Podiatrist Near You
ASG Foot & Ankle has offices in Homewood, South Chicago Heights and Mokena. Our board-certified podiatrists accept most insurance. Find the office closest to you:
Concerned About a Black Toenail?
Our board-certified podiatrists can evaluate your toenail, rule out serious causes, and get you the right treatment. Three locations in the South Chicago suburbs, with same-week appointments often available.