What to do if blood clots under a toenail?

Depends: Subungual hematoma (blood clot underneath the nail) is best treated immediately after injury or running. If it is close to the end of the nail, you could trim part of the nail and drain it. Sometimes a hot sterile poker could be used also to drain the blood. Once it has dried up then besides soaking in a warm salt bath, only time will allow it to grow off.

Nothing: Usually from trauma. If very painful, it can be drained through the nail by medical persons. If not it will resorb in time. If no trauma/ injury, one must be sure it is not a melanoma under the nail. Needs medical review.