Clinical trials do not address the best timing to start dopaminergic therapy in PD. Treatment should be tailored to the needs of each patient and delaying treatment may be an option for many of them. You should aim to maintain a good quality of life for your patient.
There are some medications to avoid because they worsen PD:
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Fahn S, Oakes D, Shouldon I, et al; Parkinson Study Group. Levodopa and progression of Parkinson’s disease. N Engl J Med. 2004;351(24):2498-2508.
Marras C, Lange AE, Eberly SW, et al; Parkinson Study Group DATATOP and PRECEPT investigators. Mov Disord. 2009; 24(16):2370-2378.