Additional Study Information: ASPIRE is a clinical trial to investigate aspirin as a treatment to improve exercise performance in people with multiple sclerosis (MS) who experience overheating during exercise. Persons with multiple sclerosis benefit from exercise, but many avoid it because of exhaustion and overheating. This randomized controlled trial (RCT) tests aspirin as a method to increase time to exhaustion for persons with MS, through its antipyretic mechanism. Participants will be seen at our laboratory for maximal exercise tests on three separate days. At each session, they will be given one of three treatments: aspirin, acetaminophen (a drug that is anti-inflammatory but not antipyretic, thereby allowing for isolation of the antipyretic action of aspirin), and placebo. Primary outcome is increased time to exhaustion, secondary outcome is reduced body temperature increase during exercise.