Verified answer

I suffered for years with irritable bowel syndrome and frequently had to make desperate dashes to the restroom. I just filled my boss in on what was up (omitting gory details) and let him know I'd stay later to make sure my full work time was fully covered. It worked out just fine. However, if this is a rare event for you, it might be better to tell him you're ill, then go to the store and get some Immodium to stop the diarrhea so you feel better and can go back to work tomorrow. I'm sure your coworkers have a general idea of what's bothering you, considering where you keep running and the general air quality in the restroom after you leave.

If it is interfering with your work that much, I would ask to be excused for the day. You might have something contagious and you don't want to pass that on to any of your coworkers.