In the studies, the most common side effects with Tysabri (seen in between 1 and 10 patients in 100) were urinary tract infection (infections of the structures that carry urine), nasopharyngitis (inflammation of the nose and throat), urticaria (rash), headache, dizziness, vomiting, nausea (feeling sick), arthralgia (joint pain), rigors (shaking chills), pyrexia (fever) and fatigue (tiredness).