This study compares the safety, effectiveness, and tolerability of MN-166 (ibudilast) to placebo (pills that looks like MN-166 capsules but contain no active medicine) in patients who are taking riluzole (an approved medication for ALS). MN-166 has been approved in Japan for almost 30 years for treatment of post-stroke dizziness and asthma. However, MN-166 has not been approved by the US Food and Drug Administration (FDA) for use in the US, Canada, and Europe. This means that MN-166 can only be used in research studies. So far, this drug has been given to more than 750 people who have participated in clinical trials including healthy volunteers, people with diabetes who have nerve pain, and people with multiple sclerosis.