Advanced-stage Parkinson's disease (PD) and myasthenia gravis (MG) share overlapping clinical features, including dysphagia, dysarthria, fatigue, and fluctuating symptoms. When PD precedes MG, these manifestations may be misinterpreted as a progression of PD, potentially leading to an underdiagnosis of MG. Although the coexistence of PD and MG is rare, it nevertheless requires careful consideration. Furthermore, the pharmacological treatments for these two disorders differ substantially, and medications used for one condition may exacerbate the other condition. We report a patient with PD complicated by MG, in whom anticholinergic treatment with biperiden led to the exacerbation of MG-related symptoms, ultimately resulting in the diagnosis of MG.