My Toenail Is Black and Thick: Causes and Treatments

A toenail that has turned black and grown thick is almost always caused by one of two things: a fungal infection that has been slowly worsening, or blood trapped under the nail after some kind of injury. These two causes account for the vast majority of cases, though a handful of less common conditions, from psoriasis to certain medications to rare cancers, can produce a similar look. The overlap between “dark” and “thick” is what makes the problem confusing, because different causes demand very different responses.

Fungal Infection Is the Leading Cause of Thick, Discolored Toenails

Onychomycosis, the medical term for a fungal nail infection, is far and away the most common reason a toenail gradually thickens and darkens. While people tend to picture a yellow nail when they think of fungus, the color spectrum is broader than that. Severely infected nails can become brown, greenish, or nearly black as layers of debris and damaged nail plate pile up. Fungal infections account for up to about 30 percent of all diagnosed superficial fungal infections and are caused by three broad groups of organisms: dermatophytes (the most common culprit in temperate climates), yeasts, and non-dermatophyte molds.¹

The infection typically starts at the tip or side of the toenail and works its way backward toward the cuticle over months or years. The nail plate thickens because the fungus triggers the nail bed underneath to produce excess keratin, the same protein the nail itself is made of. That buildup pushes the nail upward and outward, making it look swollen and deformed. As the fungus spreads deeper, the nail can darken from trapped debris, crumble at the edges, and separate from the nail bed. The process is slow enough that many people ignore it until the nail looks dramatically different from its neighbors.

Dermatophytes are almost always considered a true infection when they show up in a nail sample. Other molds, however, can be airborne contaminants that land on a damaged nail without actually causing disease, so lab results need to be interpreted carefully.¹ That distinction matters when your doctor is deciding whether to prescribe treatment or simply monitor the nail.

Trauma, Both Sudden and Chronic

A single hard stub or a heavy object dropping on your toe can rupture tiny blood vessels in the nail bed, creating a subungual hematoma, which is essentially a bruise trapped under the nail. The blood pools and turns the nail dark purple, blue, or black. When the hematoma is large, it also creates pressure that lifts the nail plate, which can make the nail feel thicker or misshapen in the weeks that follow.

Less obvious is chronic, repetitive microtrauma. Runners, hikers, and anyone whose toes repeatedly jam against the front of their shoe can develop gradual changes to the nail matrix, the tissue at the base that generates new nail growth. Chronic trauma to this area leads to pigmentary changes, ridging, and nail dystrophy, a term that simply means the nail has lost its normal shape and texture.² A runner’s toenail can end up both black from repeated micro-bleeding and thick from the matrix trying to repair itself over and over. This is why distance runners sometimes lose toenails entirely.

In the context of nail changes, distinguishing between old blood and something more concerning is one of the most important clinical questions a doctor faces. Non-continuous nail changes that affect only part of the nail are most often caused by subungual hemorrhage or onychomycosis rather than anything more sinister.³

When to Worry About Melanoma

The reason doctors take dark toenails seriously is subungual melanoma, a rare but dangerous cancer that arises in the nail matrix. It typically presents as a dark streak running lengthwise down the nail, and as it grows, the nail can thicken, deform, or partially separate. Melanoma under the nail is uncommon in the general population but is disproportionately represented among melanomas in people with darker skin tones, where it can be easily overlooked or mistaken for a bruise.

Several features help doctors distinguish melanoma from a simple hematoma. In a study of nail-unit melanomas, Hutchinson sign (dark pigment spilling onto the skin surrounding the nail), longitudinal irregular bands, and ulceration were found in the vast majority of melanoma cases but were absent in subungual hemorrhages.⁴ If your dark toenail has a sharply defined band of pigment that is widening over time, if the surrounding skin is also darkening, or if the nail is ulcerated, get it looked at promptly. A bruise, by contrast, tends to be a single blotch that grows out with the nail over several months.

A practical rule of thumb: if a dark spot under the nail does not grow out and begin disappearing within a few months, or if it appeared without any injury you can recall, that warrants a dermatology visit. Melanoma is treatable when caught early and devastating when it is not.

Psoriasis and Inflammatory Skin Conditions

Psoriasis can affect the nails independently or alongside the skin plaques most people associate with the disease. When the toenails are involved, the most common change is subungual hyperkeratosis, where the nail bed produces excess material that thickens and lifts the nail. Toenails are actually more prone to hyperkeratosis, roughening, and discoloration than fingernails in people with psoriasis.⁵ The thickened, crumbly nails that result can look very similar to a fungal infection, which is one reason misdiagnosis is common.

Nail plate abnormalities such as pits, grooves, and discoloration trace back to whether the psoriasis affects the matrix (where the nail forms) or the nail bed (the tissue the nail sits on).⁶ The affected nails often thicken and crumble, and because they are so visible, patients tend to withdraw from normal activities and social interactions.⁷ A key clue to psoriatic nails, rather than fungal nails, is the presence of pitting or oil-drop discoloration on other nails, or psoriasis elsewhere on the body. But the overlap is real, and the two conditions can even coexist.

Bacterial Infections and Extreme Thickening

A toenail that has turned black-green rather than pure black may signal a Pseudomonas aeruginosa infection, sometimes called green nail syndrome. Pseudomonas is a bacterium that thrives in moist environments and typically moves in when the nail is already damaged or separated from the bed. The discoloration ranges from greenish-yellow to greenish-black, and the nail often shows chronic, painless swelling around the cuticle. People whose hands or feet are constantly exposed to water or who have pre-existing nail trauma are at higher risk, and older adults are especially susceptible.⁸ Pseudomonas can also co-infect alongside a fungal infection, creating a combination that produces both the thickening of onychomycosis and the dark pigmentation of bacterial colonization.⁹

At the extreme end of nail thickening lies onychogryphosis, sometimes called ram’s horn nail. This is a growth disorder where the nail plate becomes opaque, yellow-brown, grossly thickened, elongated, and dramatically curved, resembling the horn of a ram.¹⁰ It tends to occur in elderly patients whose nail growth has become disorganized from years of trauma, poor circulation, or neglect. The nail may be so thick and curved that it cannot be trimmed with normal clippers and requires professional podiatric care.

Medications That Change Nail Color and Shape

Certain drugs, particularly chemotherapy agents, can cause nail discoloration, thickening, or even nail loss as a side effect. The nails are metabolically active tissue, and drugs that interfere with rapidly dividing cells do not spare them. Cyclophosphamide, for instance, has been documented to cause diffuse hyperpigmentation of the nails, dark longitudinal streaks, horizontal grooves, and nail separation from the bed.¹¹ Other agents in the same class can produce similar changes. If you notice your toenails darkening or thickening after starting a new medication, mention it to the prescribing doctor. The nail changes are typically reversible once the offending drug is stopped, though it takes many months for a toenail to grow out completely.

Getting the Right Diagnosis

Because a black, thick toenail can result from at least half a dozen different conditions, accurate diagnosis matters before committing to treatment. For suspected fungal infections, doctors typically take a nail clipping or scraping and send it for culture or molecular testing. Traditional culture has been the standard for years but can miss infections, particularly those caused by dermatophytes. Newer PCR-based testing detects dermatophytes at a higher rate than culture and correlates well with what is seen on microscopic examination of nail tissue.¹² When both PCR and microscopic examination come back negative, that combination is a reliable indicator that fungus is not the problem, which is useful because it redirects the workup toward other causes like psoriasis or trauma.

For any nail with features suspicious for melanoma, including a widening dark streak, pigment spilling onto the cuticle skin, or ulceration, the diagnostic step is a biopsy of the nail matrix. This is a minor surgical procedure performed under local anesthesia. Dermoscopy, a technique that uses a magnifying lens with polarized light, can help a dermatologist decide whether a biopsy is warranted by identifying the patterns described earlier that differentiate blood from melanoma.⁴

Treating Fungal Toenail Infections

Treatment depends on how much of the nail is affected and how thick it has become. For mild to moderate infections where less than half the nail is involved, topical antifungal lacquers or solutions applied directly to the nail are the first option, offering a better safety profile than pills. For moderate to severe cases, oral antifungal medication is generally recommended because the drug reaches the nail bed through the bloodstream, which topical treatments struggle to penetrate through a thickened nail plate.¹³

Among oral options, terbinafine is the front-runner. A Cochrane review of the available trials found that terbinafine was more effective than azole antifungals for both clearing the fungus and improving the nail’s appearance.¹⁴ A network meta-analysis that compared all monotherapy options head-to-head confirmed that daily terbinafine and daily itraconazole had significantly better odds of clearing the fungus than any topical treatment used alone.¹⁵ A typical course of terbinafine for toenails runs about three months, but because toenails grow slowly, you will not see the full cosmetic result for closer to a year. The old, damaged nail has to grow out entirely and be replaced by healthy nail.

For nails that are so thick the infection is effectively sealed off from topical medication, debridement, meaning physically reducing the nail’s thickness, helps the drugs work. Topical urea at 40 percent concentration is the standard approach for softening and thinning the nail plate. It works by breaking down the protein bonds that hold the nail together and is applied nightly under an occlusive wrap. It can be used on its own or combined with periodic nail drilling to further reduce thickness.¹⁶ Podiatrists also use mechanical burrs to thin the nail in the office, making it more permeable to whatever topical agent follows.

Laser Treatment

Laser therapy for fungal nails has gained popularity, in part because it avoids the liver-monitoring requirements that come with months of oral antifungal pills. The most commonly used devices are Nd:YAG lasers operating at 1064 nm, which penetrate the nail and heat the underlying tissue to temperatures that damage fungal cells. A meta-analysis of the published studies found an overall mycological cure rate of about 63 percent with Nd:YAG lasers and about 74 percent with CO2 lasers, though the CO2 data came from fewer studies and had wider uncertainty. The treatment is generally safe, though patients may experience tolerable pain during the session, and occasional bleeding has been reported.¹⁷

Laser outcomes appear to improve when the treatment is combined with a topical antifungal agent rather than used as a standalone therapy.¹⁸ One limitation worth knowing: laser treatment for onychomycosis is often not covered by insurance. Sessions typically run a few hundred dollars each, and multiple sessions are usually needed. The evidence is promising but still less robust than the decades of data behind oral terbinafine.

What to Do About a Subungual Hematoma

If your toenail turned black suddenly after an injury and the throbbing pain is significant, the trapped blood may need to be drained. This procedure, called nail trephination, involves creating a small hole in the nail plate to let the blood escape. It can be done with a heated wire tip (electrocautery) or even a large-gauge needle twisted gently through the nail. In a prospective study, all patients who underwent nail trephination reported relief of pain after the procedure, and no complications of infection, bone infection, or major nail deformities occurred regardless of the size of the blood collection or whether the underlying bone was fractured.¹⁹ The same study found that removing the entire nail to repair the nail bed, once the standard recommendation, was unnecessary for straightforward hematomas.

Smaller hematomas that are not causing severe pain can be left alone. The blood will be slowly pushed forward as the nail grows out over the following months. The nail may eventually fall off on its own and regrow, which looks alarming but is usually harmless as long as the nail bed was not severely lacerated.

Higher Stakes for People with Diabetes

Fungal toenail infections deserve extra attention in people with diabetes. Onychomycosis is more common in diabetics, and the consequences are more serious: the infection can serve as a portal of entry for secondary fungal and bacterial infections, contribute to foot ulceration, and in advanced cases, raise the risk of amputation.²⁰ Reduced blood flow and impaired immune responses in the feet make it harder for the body to keep a mild infection from escalating. If you have diabetes and notice a toenail changing color or thickness, treating it early is more than cosmetic. A crumbling, thickened nail can catch on socks and create small skin breaks that you may not feel if you have neuropathy, and those breaks can spiral into ulcers surprisingly fast.

The Psychological Weight of Nail Problems

People sometimes feel embarrassed about bringing up a discolored toenail at a medical appointment, as though it is trivial compared to other health concerns. But research consistently shows that nail dystrophy takes a real psychological toll. In a study comparing people with and without onychomycosis, those with the infection reported feeling socially excluded, upset, and embarrassed by their nails.²¹ The limitations go beyond vanity. Thick, dystrophic nails can make wearing certain shoes painful, interfere with exercise, and cause people to avoid situations like swimming pools or going barefoot at the beach.²²

A psychometric study found that patients with nail dystrophy rated their own functional limitations significantly higher than their doctors did, and showed markedly elevated psychological symptoms compared with matched controls. Obsessive-compulsive tendencies, anxiety, and feelings of paranoia were all higher in the nail-dystrophy group.²³ Whether the nail problems worsen pre-existing vulnerabilities or create new distress is hard to untangle, but the gap between how patients experience the problem and how clinicians rate it is worth noting. If a thick, black toenail is affecting your daily life or how you feel about yourself, that is a legitimate medical concern, not vanity.

Practical Prevention for Recurrence

Fungal toenail infections are notoriously prone to coming back, even after successful treatment. A few habits reduce the odds. Keep toenails trimmed short and straight across to minimize the surface area that fungi can colonize. Dry your feet thoroughly after bathing, including between the toes. Rotate shoes so they have at least 24 hours to air out between wearings, because fungi thrive in the warm, damp interior of a recently worn shoe. Antifungal powders or sprays applied to shoes can help, though the evidence for these is more anecdotal than rigorous.

For trauma-related discoloration, footwear fit is the single biggest lever. A shoe with a generous toe box that does not compress or jam the toes on downhill slopes prevents most of the repetitive microtrauma that runners and hikers experience. Your longest toe should have roughly a thumb’s width of space between it and the end of the shoe. Lacing technique matters too: tighter lacing through the midfoot keeps your foot from sliding forward on descents, sparing the toenails the repeated impacts that lead to blackening and eventual thickening.