Do not drink lemon water when you have diarrhea. Lemon water is made using lemons, which have a lot of acidic substances and are more irritating to the gastrointestinal system. During diarrhea, the gastrointestinal system is in a state of stress. Drinking lemon water will cause smooth muscle spasm and contraction, which will aggravate the diarrhea symptoms. Drinking plain water or oral rehydration salts at a temperature close to body temperature when you have diarrhea can replenish the water lost by the body and prevent dehydration. Those with more severe diarrhea should seek prompt medical attention.