Toenails that turn dark, thicken, crumble, or start lifting away from the nail bed are usually responding to one of a few common problems: fungal infection, physical trauma, reduced blood flow, or an underlying health condition. The cause determines whether your nail will recover on its own or needs treatment, so identifying what’s behind the change matters.
Fungal Infection Is the Most Common Cause
Fungal nail infection, known clinically as onychomycosis, is by far the most frequent reason toenails start looking dead. The fungus works its way between the nail and the nail bed, and as it goes deeper, you’ll notice a progression: first a white or yellow streak along one edge, then thickening, then the nail becomes brittle, crumbly, and misshapen. Eventually, the nail may separate from the bed entirely. A foul smell is another telltale sign.
Fungal infections typically start in a single toenail and spread to others if left untreated. They thrive in warm, damp environments like sweaty shoes, gym showers, and pool decks. If you see one discolored, thickened toenail and the rest look normal, fungus or trauma is the most likely explanation.
Treatment takes patience. Topical antifungal solutions applied directly to the nail have complete cure rates ranging from about 6% to 18%, depending on the product. They work best for mild cases. Oral antifungal medications are significantly more effective: the most commonly prescribed option achieves a complete cure in roughly 38% of cases and clears the fungus in about 70%, taken daily for 12 weeks. Even with successful treatment, toenails grow slowly, so it can take 10 to 18 months before a fully healthy nail replaces the damaged one.
Trauma You Might Not Remember
A single hard impact, like dropping something on your foot or stubbing your toe, can cause bleeding under the nail (a subungual hematoma). The nail turns dark purple or black, and if the damage is severe enough, the nail loosens and falls off within about two weeks. This looks alarming but is usually straightforward. The nail grows back on its own.
Repetitive microtrauma is sneakier. Runners, hikers, and anyone wearing tight shoes can damage their toenails gradually without a single memorable injury. The constant pressure pushes the nail into the bed with every step, eventually causing it to thicken, discolor, and separate. If your toenails started dying after picking up a new exercise routine or switching shoes, this is likely the culprit. Properly fitting footwear with a thumb’s width of space in the toe box usually prevents recurrence.
Poor Circulation and Peripheral Artery Disease
Your toenails need a steady blood supply to grow. When circulation to the feet is reduced, nails become thin, grow slowly, and can stop growing altogether. Peripheral artery disease (PAD), where plaque narrows the arteries supplying the legs and feet, is a common cause. One of the clinical signs of PAD is that toenails and leg hair simply stop growing.
Other signs of poor circulation include cold feet, leg cramping when walking, and wounds on the feet that heal slowly. PAD is more common in smokers, people over 50, and those with high blood pressure or high cholesterol. If your toenails are changing alongside any of these symptoms, the blood flow to your feet may be part of the problem.
Diabetes and Nerve Damage
Diabetes affects toenails through two connected pathways. First, high blood sugar damages the tiny nerves in the feet over time, a condition called peripheral neuropathy. This gradually destroys protective sensation, so injuries and infections that would normally cause pain go unnoticed. Second, diabetes impairs wound healing and blood flow, creating an environment where minor nail problems can escalate into serious infections.
As neuropathy progresses, nails become dystrophic, meaning thick, distorted, and fragile. The combination of lost sensation and poor healing raises the risk of ulcers forming under damaged nails, which in severe cases can lead to bone infection or gangrene. For people with diabetes, toenail changes are never cosmetic concerns alone.
Psoriasis and Thyroid Disease
Nail psoriasis can look remarkably similar to a fungal infection, with nails lifting off the bed, thickening, and discoloring. A few features help distinguish it. Psoriatic nails often develop small pits on the surface, like tiny divots pressed into the nail with a thumbtack. You may also notice reddish-brown splotches called oil spots under the nail, which don’t occur with fungal infections. And in nearly every case, someone with nail psoriasis has psoriasis plaques somewhere else on their body, since it’s a systemic inflammatory condition.
Thyroid disorders, both overactive and underactive, can also cause nails to separate from the bed. The mechanism involves how thyroid hormones regulate cell turnover in the nail matrix. If your nails are changing alongside symptoms like unexplained weight changes, fatigue, or sensitivity to temperature, thyroid function is worth investigating.
Nutritional Deficiencies
Nails are built from keratin, and producing strong keratin requires a steady supply of specific nutrients. Iron deficiency is one of the most well-documented causes of nail problems. Low iron leads to thin, ridged nails that break easily, and in more severe cases, nails can develop a concave, spoon-like shape. Vitamin B12 deficiency produces weak, brittle nails that may turn yellow or develop dark pigmentation along the nail bed.
Vitamin C supports collagen production in the tissue surrounding the nail, so a deficiency weakens the entire nail structure. Vitamin D affects calcium absorption, and since the nail plate contains calcium, low levels are linked to slow growth and brittleness. Biotin (vitamin B7) deficiency specifically causes nails to crack and split. If your toenails are deteriorating alongside other signs like fatigue, hair thinning, or frequent bruising, a nutrient deficiency could be the underlying driver.
How Long Regrowth Takes
Once the underlying cause is addressed, toenails grow back slowly. Fingernails take about 4 to 5 months to fully replace themselves, but toenails require roughly 10 to 18 months. The big toenail, being the largest, sits at the longer end of that range. Older adults tend to grow nails more slowly, so timelines stretch further with age.
During regrowth, the new nail pushing out from the cuticle should look healthy, smooth, and pink. If the new growth is also discolored or distorted, the underlying problem hasn’t been resolved. Keep in mind that a nail lost to trauma generally grows back normally without treatment, while a nail lost to fungal infection will often become reinfected unless the fungus is fully eliminated first.
Signs That Need Prompt Attention
Most toenail problems are slow-moving and not urgent, but a few situations warrant faster action. Redness, swelling, or pus spreading beyond the nail fold suggests a bacterial infection that can worsen quickly. Foul-smelling drainage, especially with a greenish tint, may indicate a secondary bacterial infection on top of an existing nail problem. If you have diabetes or any condition that reduces blood flow to your feet, even minor nail changes deserve professional evaluation, because the risk of complications is significantly higher when sensation and healing are impaired.