Among the therapeutic agents for Parkinson's disease (PD), there are important drugs that precede drug repositioning. Amantadine, developed as a treatment for influenza A, has been long used as a treatment for PD. Zonisamide is an antiepileptic drug developed in Japan, where its therapeutic effects on PD were also discovered and developed. In recent years, dabrafenib, a therapeutic agent for malignant melanoma, has bean identified as a potential therapeutic agent for PD. Amantadine and zonisamide are drugs that have been serendipitously developed in clinical settings for patients with PD. Meanwhile, the potential for repurposing dabrafenib was discovered by utilizing the results of genome-wide association studies, drug databases, and protein-protein interaction databases.