In this month’s issue of Neurology, important evidence is presented that dopamine replacement therapy is not toxic, and does not accelerate disease progression. Parkkinen and colleagues at Queen Square in London examined pathology in 96 post-mortem Parkinson’s disease brains, and paired the tissue with clinical information including levodopa use. The study concluded that in the human condition “chronic use of L-dopa does not enhance progression of Parkinson’s pathology.”

In an accompanying editorial, two prominent neurologists in the field pointed out that there “remains lingering concerns as to whether levodopa is toxic to dopamine neurons and accelerates the degenerative process.” The science quoted to support these claims has included levodopa undergoing auto-oxidation, and forming reactive oxygen species and toxic protofibrils. Additionally, the science includes a classical experiment that showed when levodopa was mixed with brain cells placed in a dish, there was toxicity.
The research, however, has fallen short in demonstrating toxicity of the drug in the human form of Parkinson’s disease. In fact, there now exists broad levels evidence from many studies across many countries (including most recently the ELLDOPA study) that levodopa is extremely beneficial to the human patient, and that levodopa has had a positive effect on disease course. (my own note here: this "recent" study was published in 1999)

Sinemet was recently reported as the most commonly administered drug among 4000+ patients being followed longitudinally in the National Parkinson Foundation Quality Improvement Initiative study. Expert practitioners who reported in this database, utilized levodopa more than any other drug-- including dopamine agonists, and they used levodopa more (not less) as disease durations increased.
(I'm not sure what the author thinks this is supposed to indicate)

Just saying.....