Background: Adherence to medication treatment in patients with multiple sclerosis (MS) is important to increase its effectiveness, reduce patient disability, prevent attacks, and increase the quality of life. Aim: This study investigated factors that influence adherence to disease-modifying therapy in patients with MS. Methods: This descriptive study was conducted with 198 patients with MS who met the inclusion criteria and agreed to participate between July 2016 and February 2017. Data were collected using an Individual Identification Form that included sociodemographic characteristics, the Multiple Sclerosis Treatment Adherence Questionnaire, the Fatigue Severity Scale, the Self-Efficacy Scale, and the Brief COPE. Results: We found that 59.6% of the patients were adherent to therapy. Patients were significantly more adherent to Avonex than other treatments, and memory problems was the most common reason for missing or forgetting medication in nonadherent patients. There was a significant difference between medication adherence and some sociodemographic characteristics and disease characteristics (P < .05). There was no significant difference between coping attitudes, fatigue, and self-efficacy level and medication adherence (P > .05). Conclusion: Patients' adherence to medication treatment was low and may be associated with social, physical, and cognitive measures.