Thousands of pending lawsuits allege that the weedkiller paraquat causes Parkinson's Disease (PD), a connection its manufacturer tried to hide from regulators and the public for many years. "Documents unveiled as part of ongoing court proceedings in California show that [the chemical company] Syngenta has spent decades investigating the potential side effects of its product," the LA Times reported in December. "This research has sometimes contradicted public statements issued by the company."

These allegations have attracted major media attention, but there's an awkward fact rarely mentioned in the headlines: a large body of research has failed to produce any solid evidence that paraquat exposure causes Parkinson's.

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Join hosts Dr. Liza Dunn and GLP contributor Cameron English on episode 248 of Science Facts and Fallacies as they break down this recent GLP feature:

* Phony Whistleblower Gambit: This is how far some environmental activist groups are willing to go to corrupt science for money and ideological gain

Only available to licensed pesticide applicators, paraquat was used in agriculture without incident for many years. Long after it was commercialized, however, scientists discovered the herbicide was chemically very similar to an impurity found in illicit opioids that caused Parkinson-like symptoms in several recreational drug users. This discovery fueled rampant speculation about the risks of paraquat exposure, though subsequent research showed that the herbicide doesn't cross the blood-brain barrier efficiently enough to cause the neurological damage at the root of PD.

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Studies have also shown that workers at paraquat manufacturing plants, individuals with the highest exposure to the chemical, are no more likely to develop Parkinson's than the general population. With this data in mind, is there any additional evidence that can help settle the dispute about paraquat and Parkinson's? Let's take a closer look at the science and politics surrounding this controversial herbicide.

Dr. Liza Dunn is a medical toxicologist and the medical affairs lead at Bayer Crop Science. Follow her on X @DrLizaMD

Cameron J. English is the director of bio-sciences at the American Cou...