Does a bruised nail cause discoloured nails?

A subungual haematoma is blood trapped between the nail plate and the nail bed. It may appear red, purple, brown or black, and the shade can change as the blood settles. Tenderness or a throbbing feeling may occur when pressure builds under the nail. As the nail grows, the discoloured area should gradually move toward the tip rather than remain fixed at the base.

A dark nail can have other causes, including infection, pigment changes or repeated trauma. Colour alone cannot identify the cause, especially when there was no clear injury.

Protect the finger or toe from further knocks, keep the nail clean and dry, and use a cool pack wrapped in cloth for short periods to ease early swelling. Avoid drilling, piercing or lifting the nail yourself. Wear roomy shoes if a toe is affected. A clinician can decide whether pressure relief is appropriate after examining the injury; this is most useful when pain is strong or the injury is recent.

Arrange prompt medical assessment if the nail is split, lifted, deeply cut or bleeding does not settle, or if the finger or toe looks bent or feels numb. Seek care if pain worsens, swelling spreads, pus develops, or the area becomes increasingly warm. A dark mark that does not grow out with the nail, or that appears without an injury, also deserves an examination.

Ask whether the nail bed or bone may be injured, whether the trapped blood needs treatment, and how to protect the nail during regrowth. Mention when the colour appeared, what caused the impact, and whether pain or numbness is changing.