Black spots under a toenail: causes and warning signs

Tight shoes, running, or stubbing the toe may cause blood to collect under the nail even when the injury seemed minor. Fungal nail change, debris, bacterial growth, or pigment produced in the nail unit can also alter colour.

Blood from trauma commonly appears red-purple, brown, or black and should gradually travel toward the nail tip as the nail grows. It may be tender soon after injury. Thickening, crumbling, lifting, odour, or debris points toward nail disease but cannot identify a specific infection by sight.

Pigment that forms a band, widens, has uneven colours, or spreads onto the skin beside the nail requires professional inspection. Nail-unit melanoma is uncommon, but it can present with a pigmented nail streak; the condition may be painless, so lack of discomfort does not rule out a concerning change.

Photograph the nail in good light and note any recent impact, long walk, sport, or tight footwear. Keep the toe clean and dry, trim only the free edge, and choose shoes with enough toe space. Do not drill, burn, or pierce the nail at home; draining trapped blood is a clinical procedure when appropriate and unsafe attempts can introduce infection.

Avoid covering an unexplained mark with polish before assessment because the colour and borders need to be visible. A clinician may examine the nail with magnification, test debris if infection is suspected, or recommend sampling when the pigment pattern is uncertain.

Obtain prompt care after a crush injury when pain is intense, the toe looks misshapen, the nail is split, or walking is difficult. Redness spreading beyond the nail, warmth, pus, fever, or worsening swelling can signal infection. Arrange a dermatology review for pigment without a clear injury, a mark that does not move outward with nail growth, a widening dark band, colour on surrounding skin, or repeated bleeding.

Ask whether the colour is blood, external material, infection, or pigment arising in the nail. Clarify whether the nail bed or bone may have been injured and how to monitor regrowth. Mention similar bands on other nails, medicines, footwear pressure, and photographs showing where the mark began.