Yes, a stomach flu can cause dehydration. This is because the virus causes vomiting and diarrhoea, which can lead to losing fluids and electrolytes. Dehydration can be a severe condition, so it is vital to stay hydrated if you have stomach flu. Here are some tips on how to stay hydrated while recovering from the stomach flu: Drink plenty of fluids. The best fluids to drink are clear liquids, such as water, clear broths, tender coconut water etc. These fluids will help replace the fluids and electrolytes you have lost. Avoid caffeine and alcohol. Caffeine and alcohol can dehydrate you, so avoiding them while recovering from the stomach flu is best.