Parkinson's disease is a naturally occurring neurodegenerative disease that was first identified in 1817, more than 100 years before paraquat was first commercialized and sold. Parkinson's affects millions of people around the world who have never used paraquat. Gene mutations are the only known cause of Parkinson's.
Syngenta rejects the claims of a causal link between paraquat and Parkinson's disease because it is not supported by scientific evidence. Despite decades of investigation and myriad epidemiological and laboratory studies, no scientist or doctor has ever concluded in a peer-reviewed scientific analysis that paraquat causes Parkinson’s. Nor has any medical textbook or treatise concluded that paraquat causes Parkinson’s. In short, the hypothesis that paraquat causes Parkinson’s is not accepted in the medical community or peer-reviewed science, nor has it been accepted at any time in the past. In fact, according to the peer reviewed literature: there is a “consensus in the scientific community that the available evidence does not warrant a claim that paraquat causes Parkinson’s disease.” (Weed 2021) at 180.
The Weed publication, in the peer-reviewed journal Neurotoxicology, studied epidemiological meta-analyses between 2006 and the present. Dr. Douglas Weed, a physician and epidemiologist with over 25 years of experience in epidemiological research with no ties to Syngenta, concluded following a review of the scientific literature, “No author of any published review stated that it has been established that exposure to paraquat causes Parkinson’s disease, regardless of methods used and independent of funding source.”
The Agricultural Health Study (“AHS”), which is sponsored by the NIH and several independent public health institutions, including the National Institute of Environmental Health Sciences and National Institute for Occupational Safety and Health, has followed more than 66,000 chemical applicators and their spouses since 1993. In 2020, using data from the AHS study, Drs. Srishti Shrestha and Dale P. Sandler, both with the National Institute of Environmental Health Science’s Epidemiology Branch, published their 25-year update, focusing specifically on a potential link between the use of pesticides and herbicides (including paraquat) and Parkinson’s. (Shrestha 2020). That study found no statistically significant link between paraquat and Parkinson’s, and in fact, did not find any increased risk of Parkinson’s with the increased use of paraquat.
These views are supported by recent thorough reviews performed by the science-based regulatory authorities in Australia and the United States.