Does anyone have experience using Relenza to treat or prevent the flu? ## Hello, Ldkat! How are you? I've never taken it, I usually opt for the flu shot each year, but I can tell you more about it. It is an antiviral medication and if taken, before you actually have flu symptoms, it can sometimes prevent it entirely. Otherwise, if taken after you have it, it helps prevent the virus from replicating in your body, which can lessen the symptoms, their severity and the duration of the the flu. The most common side effects, as listed by the FDA, may possibly include diarrhea. nausea, headache, dizziness and cough. Are there any other questions or concerns?