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More about ABC Nightline's segment on paraquat

ABC Nightline recently broadcast a segment about paraquat, a product used to control weeds, which contained serious allegations that paraquat causes Parkinson’s disease.

Syngenta categorically denies these allegations, based both on its own scientific analysis and the independent scientific literature. While it is heartbreaking to see people suffer from Parkinson’s disease, no peer-reviewed scientific analysis has ever established a causal relationship between a farmer’s use of paraquat and Parkinson’s disease.

The ABC report was not a balanced or fair discussion of the issue. Rather, it highlighted a plaintiff’s lawyer and activists positioned as experts and incomplete scientific results to suggest causation, which is simply not true based on the peer-reviewed scientific literature. Without being able to establish causation, the report focused on emotional stories of patients with Parkinson’s, and used innuendos to position Syngenta and the US regulatory authority as villains.

We appreciate your interest in this complex issue, and invite you to read our responses to some of the points made in the program, and to reach your own conclusions.

Fast Facts

Dr. Cory-Slechta had agreed in a respected scientific publication that there was insufficient evidence of even an association between paraquat with Parkinson’s disease.

ABC Nightline included extensive commentary from Dr. Deborah Cory-Slechta, a researcher who has conducted high-dose injection studies in mice. Even lead plantiffs' counsel in the federal lawsuits against Syngenta have recently conceded to the court that these types of animal studies cannot be used to prove causation: “We don't believe, under the science that exists in this case, you can use animal studies to prove causation.” 08/21/23 Tr. at 16. While Dr. Cory-Slechta nevertheless suggested on the ABC program that paraquat is a risk factor for Parkinson’s disease, in 2009 she was one of a number of scientists independent of Syngenta who published a consensus statement finding there was insufficient evidence of even an association between paraquat and Parkinson’s disease, let alone causation.

Syngenta also acted properly with regard to Dr. Cory- Slechta’s nomination to a Scientific Advisory Panel. In the 2000s, Dr. Mona Thiruchelvam was one of the researchers using the high-dose mouse injection method discussed above. In 2007, after one of Dr. Thiruchelvam’s colleagues became suspicious of her research, her employer initiated an investigation during which Dr. Thiruchelvam provided investigators with 293 data files that were later determined to be corrupted and falsified.  

Soon thereafter, the U.S. Department of Health and Human Services’ Office of Research Integrity (“ORI”) found that Dr. Thiruchelvam intentionally falsified data used in her paraquat publications and presentations. As a result of her misconduct, Dr. Thiruchelvam entered into a voluntary exclusion agreement, which required her to retract two federally funded papers and effectively barred her from receiving federal funding for seven years.

Dr. Thiruchelvam’s fraud had widespread effects and seriously compromised a significant body of Parkinson’s research. By the time her fraud came to light, her tainted and retracted studies had already been cited in at least one hundred other papers, and her participation in other non-retracted studies called into question those results. The full extent of misconduct by Dr. Thiruchelvam remains unknown. 

Dr. Cory-Slectha was Dr. Thiruchelvam’s supervisor, mentor, and co-author. The two have co-authored at least 29 publications together based on searches of the PubMed academic database. Indeed, Dr. Cory-Slectha is a co-author on some of the very papers that the ORI retracted as part of Dr. Thiruchelvam’s sanction and that Australian regulators declined to consider as potentially fraudulent. (APVMA 2016: Supp. II) at 12. And Dr. Cory-Slectha was the head of the lab where the fraud occurred.

No scientist or doctor has ever concluded in a peer-reviewed publication that paraquat causes Parkinson's

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.


In the US, mass tort litigation has become a multi-billion dollar industry for plantiffs' lawyers as they target manufacturers of everything from pharmaceuticals and medical devices to herbicides and military grade ear plugs. Syngenta is among the latest targets of the 'Mass Tort Machine' in the U.S. as plaintiff law firms allege that people have been harmed by a product used to control weeds. The firms advertise heavily on television, radio, online and with mailings (spending tens of millions of US dollars). Those advertisements are often misleading or make false claims, such as suggesting that gardeners or golf course players can be exposed to paraquat when it is – in fact – illegal to use paraquat in either environment. The goal of these lawyers is to sign up enough claimants, (often in the thousands) to then pressure companies that manufacture or sell the product to settle the cases out of court for staggering amounts. The law firms keep 30 to 40 percent and divide the remainder among the claimants. Using this strategy, law firms can make a significant amount of money without ever having to prove their clients were harmed.

Syngenta estimates that there have been more than 200 law firms in the U.S. engaged for more than two years to recruit as many people as they can who say that they have Parkinson's disease and used paraquat.

  • $40 million - estimate that law firms have spent on television advertising over a two-year period. This does not include expenses for radio, online and direct mail advertising.
  • Misleading content - images in much of the advertising is misleading. Paraquat would not be applied to healthy crops and is not allowed, by regulation, to be applied on U.S. golf courses.
  • Dead plaintiffs - more than 20 plaintiff law firms improperly filed dozens of actions on behalf of already-deceased plaintiffs, served sworn questionnaires to the Court on behalf of deceased plaintiffs without disclosing their deaths, and apparently even copied the signatures of deceased plaintiffs to make it appear they were attesting to facts after their deaths. This is pervasive misconduct and given the limited information available to Syngenta, it is likely these discoveries are only the tip of the iceberg.
  • 300+ claimants - dismissed due to lawyers' failure to properly vet them - including bellwether cases.
  • American Tort Reform Association, an organization Syngenta has long supported, has more information about the Mass Tort Machine's practices:
    ATRA website The Hidden Money Behind the Litigation

Here are links to interesting news media articles:

Our communications with ABC Nightline

We met with the producers in New York multiple times and exchanged a number of phone calls and emails with them to provide accurate information about 60 years of scientific studies, conducted both independently and by Syngenta, as required by law and regulatory authorities in multiple countries.

Please find here information we've shared with the producers of ABC Nightline:

Our letter to ABC Nightline's producer

Scientific studies conducted both independently and by Syngenta



To examine the extent to which a consensus exists in the scientific community regarding the relationship between exposure to paraquat and Parkinson's disease, a critical review of reviews was undertaken focusing on reviews published between 2006 and the present that offered opinions on the issue of causation.

"No author of any published review stated that it has been established that exposure to paraquat causes Parkinson’s disease."
"A consensus exists in the scientific community that the available evidence does not warrant a claim that paraquat causes Parkinson’s disease"
Read more Download the report here


This 2021 study looked at 968 people in the U.K. who worked at paraquat manufacturing factories between 1961 and 1995. This study showed no increased risk of Parkinson's disease, and no increased risk of mortality from any other cause.

Read more


This 2020 study looked at U.S. farmers and their spouses over a 20-year period and found no conclusive evidence that paraquat exposure was associated with Parkinson's disease, although individuals with head injuries and paraquat exposure seemed slightly more likely to develop Parkinson's. The Environmental Protection Agency points to this study as evidence that no association exists between paraquat use and Parkinson's, although the study itself appears to point in a less certain direction.

Read more

Paraquat remains an essential tool for farmers

After 60 years, the herbicide paraquat remains essential for many farmers. Below are comments from farmers and experts, on multiple continents, in their own words. These videos were unscripted and the interviewees were not compensated for their comments.

Mark FowlerAustralia
Andrew WeidemannAustralia
Larry SteckelUSA
Tim DurhamUSA
Zaenal ArifinIndonesia
Prof. Nanik SuryaniIndonesia
Phil BothamPrincipal Science Advisor for Product Safety at Syngenta