A black toenail or 'subungual haematoma' results from the nail being compressed onto the nail bed. Symptoms: Inflammation, bruising and, sometimes, a build-up of blood under the nail. This happens most commonly with the big toe. Common causes: Shoes that slip around, allowing the toes to hit the end of the toe box or lots of downhill running when the foot slips forward in the shoe. Overly long or badly cut toenails. What to do: If there is no blood, just leave the nail to settle and grow out (it will most likely fall off at some point). If there is painful pressure caused by blood behind the nail, see a medical expert to pierce the nail and drain it. To prevent it happening again, make sure your shoes fit well and are laced up to keep the foot secure, and cut toenails short and straight across.