Why does a bruised nail cause discoloured nails?

A knock, crush injury, tight footwear, or repeated pressure can damage tiny blood vessels under the nail. Blood collects in the confined space beneath the nail plate, producing a red, purple, brown, or nearly black area. The colour can look darker as the blood changes. The mark usually moves toward the nail tip as the nail grows, although a badly injured nail may loosen or shed.

A dark nail has other possible causes, including infection, pigment changes, or a growth. A clinician may assess whether the mark follows the nail’s growth and whether there was an injury, rather than relying on colour alone.

Protect the finger or toe from further pressure, keep it clean, and use a cold pack wrapped in cloth for short periods soon after an injury to limit swelling. Do not pierce or drain the nail yourself. A clinician may consider drainage when pressure is severe or the injury is extensive, and may check for a fracture or nail-bed damage. Well-fitting shoes can reduce repeated pressure on a bruised toenail.

Seek prompt assessment for severe throbbing pain, a nail that is split or lifted, a deep cut, numbness, deformity, or difficulty moving the finger or toe. Arrange review if the dark area does not move outward as the nail grows, appears without an injury, or involves a changing streak in one nail. Increasing redness, warmth, swelling, or drainage can indicate infection.

Ask whether the nail needs drainage or imaging, how to protect the nail bed, what growth changes to expect, and which signs would suggest infection or a cause other than trauma.