Side effects that may occur while taking this medication include: Dizziness, Tiredness, Weakness, Drowsiness, Upset stomach or stomach pain, Diarrhea, Muscle weakness or aches, Muscle stiffness, Numbness or tingly feeling in or around your mouth, Mild nausea, Constipation, Headache, Runny nose, Loss of appetite, Headache Stop using Riluzole and call your doctor at once if you have any serious side effect such as: Nausea, Stomach pain, Low fever, Loss of appetite, Dark urine, Clay-colored stools, Jaundice, Fever, chills, body aches, and other flu symptoms, Chest discomfort, Dry cough, Trouble breathing, Cough with yellow or green mucus or stabbing chest pain