A 67-year-old female patient has experienced right hand tremors and slow walking for 4 years. After neurological examination, Parkinson's disease was suspected. Symptoms initially improved with medication but later worsened. Additional drugs including trihexyphenidyl, amantadine, bromocriptine, and diphenhydramine were introduced. Recently, symptoms suddenly fluctuated between exacerbation and relief, accompanied by dyskinesia. Which drug's side effect is this phenomenon?