Diarrhea is not usually a dangerous problem but can ruin your race and be quite annoying.

Causes

Similar to vomiting this can be related to your body working to move your muscles and not digest properly in combination with dietary intake. 
– Too much fiber or too much of particular sugars can lead to this so adjust your intake accordingly.
– There may also be individual food intolerance (dairy, gluten, etc.) that you do not notice in daily life but are more pronounced under the stress of a long run. Test your race day nutrition to help prevent this

Treatment

Over the counter medications
Over the counter medications such as Pepto Bismol or loperamide may help. NOTE: some medications such as Pepto can turn your stool black.

Appropriate hydration
Focus on appropriate hydration and salt intake if you are having diarrhea.

Red Flag

If you have bloody diarrhea you should stop. There is a particular type of intestinal inflammation and damage that occurs with running and produces bloody stool known as runner’s colitis. It is thought to be caused by prolonged decreased blood flow to the intestines.  There is not usually long term damage if you do not continue to push on in spite of the bloody diarrhea.  However you should follow up with your doctor if this occurs.