If you are talking about diarrhea with influenza, which is the respiratory flu, then it is usually for only a day or maybe two and it is not at all among the symptoms for many people. Some people have reported it as a symptom of the pandemic Swine Flu H1N1, but it usually is very short term.
If you are talking about the "stomach flu", which really isn't the flu at all-it is just called that-- it is usually a viral gastrointestinal illness and then often it is a three or four day illness. If it continues beyond that, you should check with your health care professional to see if an examination or prescription is needed.
There are over the counter medicines for diarrhea, ask your pharmacist for suggestions for your specific symptoms. If diarrhea continues for more than three or four days, the possibility of dehydration, especially in the very young, very old or infirm, is serious and needs to be avoided. Drinking plenty of fluids is important and if you also are nauseated or vomiting, replacing the lost fluids is critical and you may need prescription medicines to keep the fluids down.
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If you are going to work or school. then you should wait until it wears off. Maybe go and see a doctor about it if it doesn't go within about 3 days.