What to do about my badly bruised toenail?

Is it painful?: If its painful you can see your doctor and they can drain it for you. Otherwise do nothing and it will heal with time.

See a podiatrist: If you are runner, this is a common injury called "black toe syndrome" due to blood bruises under the nail plate. Other causes of black toenails are infections - either bacterial or fungal. Another cause may be from a fracture in the last bone of the toe. In any case, see a podiatrist who can examine and treat any of these conditions and recommend ways to prevent recurrence.