Cold water that has been used for pasta will generally not give you diarrhea if it has been boiled and then left to cool, but if it has been unboiled, it may give you diarrhea.
Generally speaking, tap water is purified and sterilized before it is delivered to households, and in most cases it is free of germs and parasites. However, in the process of conveying domestic water, there may be contamination. Therefore, if you use water directly from the faucet, you may contaminate the noodles, which may cause intestinal infections and diarrhea after consumption.
Tap water in the process of boiling, equivalent to the water may exist in the pathogenic bacteria for the second disinfection, the use of boiled and cooled water over the noodles generally will not diarrhea. However, people with weak gastrointestinal function are not recommended to eat cold noodles to avoid causing abdominal pain and diarrhea. When discomfort occurs, actively go to the hospital for diagnosis and treatment.