Although effective disease-modifying treatments (DMTs) are available for individuals suffering from multiple sclerosis (MS), many patients fail to take their recommended medications. Unlike medications that provide immediate relief from existing symptoms, DMTs decrease the probability of future symptoms (i.e., a probabilistic benefit) while concurrently carrying an appreciable risk of immediate side effects (i.e., a probabilistic cost). Prior research has shown that both the probability of reducing disease progression and the probability of experiencing side effects impact patients’ likelihood of taking a hypothetical DMT. The role that side effect severity plays in treatment decisions remains unexplored. The present study examined how probability of medication efficacy and side effect severity impact patients’ likelihood of taking hypothetical DMTs. Patients’ likelihood of taking a DMT systematically decreased as medication efficacy decreased and side effect severity increased. Because side effect severity appears to impact decision-making processes in unique ways, the present results suggest that providers should present information on severe (which are typically rare) and mild to moderate side effects (which are more common) separately.