The most common side effects of TAMIFLU when used for treatment of the flu include nausea, vomiting, and headache. The most common side effect of TAMIFLU when used for prevention of the flu include nausea, vomiting, headache, and pain. Tell your healthcare provider if you have any side effect that bothers you or that does not go away. These are not all of the possible side effects of TAMIFLU. Call your doctor for medical advice about side effects.