Guinea pig has diarrhoea, what can i do?

Until you can seek out a vet, give them uncooked oatmeal to help bind them. They need Electrolytes like in Pedialyte ( they like orange flavor). You can give them a vitamin - c powder in their water but be sure to change it frequently. Tetracycline powder (fish capsule medication) is the only medications our vets have used on guinea pigs for us. Continue small amounts of their regular grain, but no fruits or veggies. Bread is another binder for diarrhea.