Black Toenail and Diabetes: Causes, Warning Signs, and When to Seek Help
Noticing a black toenail can be alarming, especially if you have diabetes. While dark discoloration under the nail may seem harmless, it can sometimes signal injury, infection, or circulation problems — all of which require special attention in people with diabetes.
This article explains what causes black toenails in diabetes, when to worry, and how to protect your feet to prevent complications.
Why Black Toenails Happen
A black toenail occurs when blood, pigment, or debris collects under the nail. It may result from trauma, fungal infection, or poor circulation. In most people, it’s minor and heals with time. In someone with diabetes, however, the same issue can lead to infection, ulceration, or even tissue damage if untreated.
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Common Causes of Black Toenails in Diabetes
1. Nail Trauma (Subungual Hematoma)
Even small injuries — such as stubbing your toe or wearing tight shoes — can cause bleeding under the nail. The trapped blood appears black or purple.
Because diabetes can cause reduced sensation (neuropathy), many people don’t feel the injury happen. The damaged area may go unnoticed until discoloration appears.
2. Fungal Nail Infection (Onychomycosis)
People with diabetes are more prone to fungal infections due to high blood sugar levels that create a hospitable environment for fungi.
Signs include:
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Yellow, brown, or black discoloration.
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Thickened, brittle nails.
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Foul odor or debris under the nail.
If untreated, fungal infections can spread and lead to painful inflammation or bacterial infection.
3. Poor Circulation (Peripheral Arterial Disease)
Diabetes can narrow blood vessels in the feet, reducing oxygen supply to tissues. This poor circulation slows healing and increases the risk of nail and skin discoloration.
A black toenail may sometimes be an early warning sign of tissue damage (gangrene), especially if the area feels cold or numb.
4. Bacterial Infection or Ulcer Formation
Cuts or sores under or around the toenail can become infected if unnoticed. If infection spreads to deeper tissues, the nail may darken due to necrosis (dead tissue).
5. Melanoma (Rare but Serious)
A dark streak or spot under the nail that doesn’t grow out or respond to treatment could be subungual melanoma, a rare type of skin cancer. Always have persistent or spreading discoloration examined by a doctor.
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How Diabetes Affects Toenail Health
People with diabetes are more vulnerable to foot problems because of two major complications:
1. Peripheral Neuropathy
High blood sugar damages nerves over time, causing numbness or tingling. As a result, injuries often go unnoticed, allowing infections or bruises to worsen before being treated.
2. Poor Circulation
Reduced blood flow makes it harder for wounds or nail injuries to heal. Even minor nail trauma can turn into an infection if blood supply is limited.
Together, these issues make proper foot care and early detection essential.
Symptoms to Watch For
While a black toenail might just be a bruise, it’s important to look for signs that could indicate infection or poor circulation.
Seek medical attention if you notice:
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Pain, swelling, or warmth around the nail.
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Pus or drainage.
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Thickened or brittle nails.
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Foul odor.
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Cold, pale, or numb toes.
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Black areas that spread or blister.
Any of these could signal an infection or circulation problem needing urgent treatment.
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Diagnosis
Your doctor or podiatrist will examine the nail and surrounding skin. They may perform:
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Nail culture to check for fungal infection.
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Blood flow studies (such as ankle-brachial index testing).
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Blood sugar and A1C tests to assess diabetes control.
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Imaging (X-ray or ultrasound) if deeper tissue damage is suspected.
Early diagnosis prevents complications that could lead to ulcers or, in severe cases, amputation.
Treatment Options
The treatment depends on the cause of the discoloration:
1. For Bruising or Injury
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Keep the area clean and dry.
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Avoid tight footwear.
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Trim nails carefully or see a podiatrist.
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If the nail is lifting or causing pain, your doctor may drain the blood or remove the nail.
2. For Fungal Infections
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Topical antifungal creams or medicated nail lacquers.
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Oral antifungal medications (under medical supervision).
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Laser treatments or debridement for stubborn cases.
3. For Poor Circulation
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Manage blood sugar, blood pressure, and cholesterol.
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Quit smoking to improve blood flow.
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Exercise regularly (with doctor approval).
4. For Bacterial Infections
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Oral or topical antibiotics.
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Wound care or debridement.
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Hospitalization if infection spreads to bone or tissue.
5. For Melanoma
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Referral to a dermatologist for biopsy and treatment.
Preventing Black Toenails in Diabetes
Foot care is a cornerstone of diabetes management. Preventing nail injuries and infections can save you from serious complications.
Prevention tips:
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Inspect your feet daily for color changes, cuts, or swelling.
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Trim nails straight across and avoid cutting too short.
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Wear well-fitting shoes with breathable materials.
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Keep feet clean and moisturized, but avoid excess moisture between toes.
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Never walk barefoot.
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Schedule regular foot exams with a podiatrist.
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When to See a Doctor
Call your healthcare provider or podiatrist if you have diabetes and notice:
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A black toenail that doesn’t improve within a few weeks.
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Pain, swelling, or drainage.
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A foul smell from the toe.
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Spreading discoloration or blackened skin.
Early treatment can prevent infection, ulcers, and tissue loss.
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The Bottom Line
A black toenail in diabetes is often caused by trauma or infection, but it should never be ignored. Because diabetes affects both sensation and healing, even minor nail changes can become serious if untreated.
Checking your feet daily, wearing proper shoes, and managing your blood sugar are the best ways to protect your toes — and your long-term health.
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References
American Diabetes Association. (n.d.). Peripheral neuropathy. https://diabetes.org/about-diabetes/complications/neuropathy/peripheral-neuropathy
Cleveland Clinic. (2024, September 26). Subungual hematoma: Bleeding & bruising under the nail. https://my.clevelandclinic.org/health/diseases/subungual-hematoma
Mayo Clinic. (2025, June 10). Diabetic neuropathy – Symptoms & causes. https://www.mayoclinic.org/diseases-conditions/diabetic-neuropathy/symptoms-causes/syc-20371580
Navarro-Pérez, J., García-Klepzig, J. L., Lázaro-Martínez, J. L., & Aragón-Sánchez, J. (2023). Onychomycosis associated with diabetic foot syndrome: A systematic review. Mycoses, 66(6), 430-444. https://doi.org/10.1111/myc.13577
Palla, A. R. (2024). Diabetic peripheral neuropathy. In StatPearls. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK442009/
Salam, R. A., & Supriya, R. (2021). Diabetes and peripheral artery disease: A review. World Journal of Diabetes, 12(6), 827-838. https://doi.org/10.4239/wjd.v12.i6.827
Sivanmaliappan, T. S., & Sevanan, M. (2011). Treatment of onychomycosis in the diabetic patient population. Journal of Drugs in Dermatology, 10(11), 1178-1181. https://www.sciencedirect.com/science/article/abs/pii/S105687270200199X
Winston, J. A., & Miller, J. L. (2006). Treatment of onychomycosis in diabetic patients. Clinical Diabetes, 24(4), 160-166. https://doi.org/10.2337/diaclin.24.4.160
Sequeira, M. B., & Launico, M. V. (2025, June 24). Subungual Hematoma Drainage. Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK482508/
