Introduction: Multiple sclerosis (MS) is a chronic inflammatory, demyelinating and neurodegenerative disease of the central nervous system. MS has no curable disease but drug modifying therapy (DMT) can delay the long-term disability progression of the disease. The effectiveness of MS treatment depends on the patient’s adherence to therapy. Aim: This study evaluated the level of intentional non-adherence and the relationship between beliefs about medication and the level of intentional non-adherence to treatment of patients with multiple sclerosis. Material and methods: A group of 146 patients with relapsing–remitting MS were included. To assess different aspect of adherence, the Intentional Non-Adherence Scale (INAS) was used. For evaluating patients’ beliefs and opinions regarding medication, the Beliefs about Medicines Questionnaire (BMQ) was used. Results: The mean total INAS score was 51.41 ± 27.83 points. Patients were most concerned about the necessity to take medication and least concerned about the harm caused by medication. The overuse and harm domains of the BMQ were significantly correlated with INAS scores (p < 0.05). Conclusions: Independent determinant of intentional non-adherence was overuse.