Black Toenail: Bruise, Fungus, or the One That Needs a Dermatologist
A toenail that has gone dark is usually blood. You stubbed it, dropped something on it, or ran a long downhill in shoes that let your foot slide, and the nail bed bled under a plate that gives the blood nowhere to go. It hurts for a few days, then over months it grows out or the nail lifts and comes off.
This guide covers that story, the two look-alikes, and the one that matters: a brown or black band running the length of one nail, new, with no injury you remember, in a man over 50. Subungual melanoma looks like this, and it is not a wait-and-see.
Not sure which nail problem you have? The 5-minute self-check sorts the common ones.
What a bruise under the nail is
Subungual hematoma is the medical name: blood trapped between the nail plate and the nail bed. The plate is sealed at the edges, so the blood pools, pressure builds, and a fresh one throbs.
One blow or many. A dropped tool or a stubbed toe is obvious. The version most men over 40 have is runner's toe: the toe taps the front of the shoe on every stride and blood collects without one memorable injury, usually under the longest toe. Three things cause it: a shoe a half size short, so the toe reaches the end of the toe box; long downhills, where the foot slides forward on every step; and laces tied loose, so the heel is not held and the foot moves regardless of length.
The color sequence. Red or purple the first day, then blue-black, then flat black or brown as the blood dries. Over the following months it moves toward the tip, and a crescent of clean pink nail appears behind it. That moving edge is the most useful sign that what you have is blood.
When to drain it and when to leave it
A fresh hematoma that is very painful, covers a large share of the nail, and is less than a day or two old can be drained by a clinician. A podiatrist or urgent care clinic makes a small hole in the plate with a sterile tool and the pressure is gone in minutes.
Not at home. The hot-paperclip method is on every running forum and it is a bad idea: an unsterile tool, a burn, and an open path for bacteria into a wet space under the nail. In a man with diabetes or poor circulation it is a real risk.
When there is nothing to drain. After a few days the blood has clotted and draining does not help; a hematoma that does not hurt never needed it. If you cannot bear weight or the toe looks bent, ask for an X-ray; a fracture under the nail changes the care.
When the nail lifts and falls off
If the bleeding covered roughly half or more of the nail, the plate often separates from the bed and comes off over the following weeks. The new nail is already growing behind it.
Protect the bed. Do not pull the old nail. Leave it attached as long as it will stay and trim the loose parts as they lift. When it comes off, the bed is tender for a week or two. Keep it clean, dry, and covered until it is no longer sore.
How long to grow back. Toenails grow about 1.5 to 2 millimeters a month in healthy adults, slower with age and reduced circulation. Expect 6 to 12 months for most toenails and up to 18 months for a big toenail, the figure NHS guidance uses.
The two look-alikes that are not blood
Fungus with dark debris. Fungus turns a nail yellow, white, or brown and thickens it; it does not go flat black. But the debris packed under a fungal nail can look dark. The tell: fungus starts at the tip or side, creeps back over months, and thickens the plate, while a bruise stays flat under a normal-thickness plate and grows out. The toenail fungus self-check covers the pattern, and a clipping test at the podiatrist settles it.
Green-black under a lifted nail. When a nail has lifted from the bed, the gap holds moisture and a bacterium called Pseudomonas can move in. The color is a green-black stain on the bed under the lifted part. It is neither a bruise nor fungus: trim the nail back to where it is attached, keep it dry, and see a podiatrist.
The one that needs a dermatologist
A brown or black band that runs from the cuticle to the tip of the nail is called longitudinal melanonychia. Most bands are harmless: a mole in the matrix, a medication stain, or a normal finding in men with darker skin, who often have bands on several nails. But subungual melanoma, which starts under the nail, produces exactly this band, and it is the black toenail worth an appointment this week.
The features that raise concern:
- New, in a man over 50, with no injury he remembers.
- One nail only, most often the big toe or the thumb.
- Widening, wider than about 3 millimeters, or darker and less even at one edge.
- Pigment spreading onto the skin at the cuticle or nail fold. This is Hutchinson's sign, the feature dermatologists take most seriously.
- A nail that is splitting, cracking, or lifting where the band runs.
The "does it move" test. A bruise is blood under the plate, and the plate carries it forward. A melanoma streak comes from cells in the matrix at the base that keep producing pigment, so it stays anchored at the cuticle while the nail around it grows. For a dark patch you cannot tie to an injury: photograph the nail in daylight, then draw a fine line across it with a permanent marker at the base of the dark area. Check at four weeks and again at eight. A bruise will have moved toward the tip with the marker line; a patch still touching the cuticle after eight weeks has not moved, and it needs a dermatologist.
The test is for a patch, not a band. A new streak the length of one nail should be seen without waiting the eight weeks. The dermatologist checks it with a dermatoscope and, if in doubt, takes a small biopsy of the matrix. Caught early it is treatable; caught late it is one of the more dangerous melanomas.
Shoe-fit fixes for runners
Most runner's toe is a fit problem, and the fix is cheap.
Length. A thumb's width between your longest toe and the front of the shoe, standing, in the socks you run in, measured in the afternoon when the foot is largest.
Lace lock. Use the extra top eyelet. A heel-lock lacing (the loop-and-cross through the last pair of eyelets) holds the heel in the back of the shoe so the foot cannot slide forward on descents.
Toenails trimmed before long runs. Straight across, short enough that the nail does not reach past the toe pad, filed smooth. The ingrown toenail guide shows the trim. The rest of the shoe checklist is in runners over 40: foot pain.
What to do this week
- Decide which of the four it is. Injury you can name and a patch that moves: bruise. Thick, crumbly, started at the tip: fungus. Green-black under a lifted edge: bacterial. Band from cuticle to tip, one nail, new: dermatologist.
- Photograph it in daylight and mark the base with a marker line if you are tracking a patch.
- Book the right appointment. Painful fresh bruise: podiatrist or urgent care today. Band: dermatologist this week. Fungus or bacterial: podiatrist, routine.
- Fix the shoe if it is runner's toe: length, lace lock, nails trimmed the night before a long run.
- Keep the nightly check going. The daily routine takes a minute and catches the next one early.
When it's not a wait-and-see
Same-week appointment, not a routine one, if any of these apply:
- A brown or black band the length of one nail that is new, widening, or spreading onto the skin at the cuticle.
- A dark patch that has not moved toward the tip in 8 weeks.
- A dark nail that is splitting, bleeding, or lifting without an injury.
- A fresh, very painful hematoma under most of the nail, or an injury where you cannot bear weight.
- Red, hot, swollen skin around the nail, pus, or a smell from under a lifted nail.
- You have diabetes or poor circulation and any nail has gone dark, lifted, or fallen off.
- Redness spreading up the foot, or a fever with any foot problem.
Bottom line
A black toenail with an injury behind it is blood under the plate: it grows out, or the nail comes off and regrows in 6 to 12 months, up to 18 for the big toe. A brown or black band the length of one nail that is new, widening, or spreading to the cuticle, in a man over 50 who does not remember an injury, is a dermatologist visit this week, because a bruise moves with the nail and a melanoma does not. Fix the shoe fit, and if the nail turns out to be fungus, toenail fungus treatments, compared covers what to do next.
