I've had Immodium work within 15 minutes. My sister said it was supposed to kick in and work after 15 minutes. But other times it didn't start working for several hours. I think it depends on how extreme your situation is and on what caused it. Like if it's food poison, it has to run it's course and purge your system, so the Immodium won't work in that case. It may slow it down a little but not stop it. Pectin from apples will help stop diarrhea. So, eat applesauce. It is better than raw apples or apple juice. Bananas help, too and they restore the potassium you may lose with being sick.