You may experience some or none of the Symmetrel side effects listed in this article. Unfortunately, it is difficult to tell whether any particular side effect (such as fatigue) is actually caused by Symmetrel or other factors (such as the flu itself). Therefore, make sure to let your healthcare provider know if you develop any side effects while taking the drug or if something "just does not seem right." While it may not be a side effect of Symmetrel, your healthcare provider will be able to diagnose and treat the problem.