Top neurologists say they were snubbed in review of controversial weed killer

Australia's top neurologists say they have been ignored by the chemical regulator as it considers the future of a controversial weed killer linked to Parkinson's disease.

Dozens of the country's top movement disorder specialists say the Australian Pesticides and Veterinary Medicines Authority (APVMA) failed to engage with them during its review.

It has raised fears that "real-world data" on paraquat's link to Parkinson's disease is being overlooked.

It comes as a final regulatory decision on the herbicide is expected mid-year.

Associate professor Wesley Thevathasan, a movement disorders specialist, said he and other experts had made submissions to the regulator during the public consultation process, but had received no further contact.

"I've heard nothing from the APVMA,"

Dr Thevathasan said.

Dr Thevathasan is a Parkinson's disease specialist, and works across several hospitals, the University of Melbourne and The Florey Institute.

"We put in submissions, myself, other scientific bodies, groups of neurologists, some of the leading scientific minds and organisations in Australia who have interest in Parkinson's disease, and as far as I know, none of us have been contacted by the APVMA."

Paraquat and the related herbicide diquat are fast-acting weed killers that farming groups say are essential for controlling resistant weeds.

The chemicals have been under review by the APVMA since 1997, and they are banned in the EU, the UK and China due to their toxicity.

In its 2024 proposed regulatory decision the APVMA said "the available epidemiology data is insufficient to conclude any association between paraquat exposure and neurotoxicity (including Parkinson's disease) in the occupational environment."

It's a position that Dr Thevathasan said conflicted with the "global consensus" in his field that exposure to paraquat was closely linked to the later development of Parkinson's disease.

He said the evidence base used by the APVMA to date gave "almost no weight" to recent human studies.

"There are now multiple epidemiological studies or real-world exposure studies that have suggested that paraquat increases the risk of Parkinson's disease by around threefold,"

Dr Thevathasan said.

Parkinson's disease is a progressive, incurable neurological condition that affects movement and can cause stiffness and uncontrollable shaking.

Dr Thevathasan is not the only expert to raise concerns. More than 70 medical experts and clinicians from major hospitals, research institutes and movement disorder clinics around Australia have also put their names to submissions highlighting evidence linking paraquat to Parkinson's disease.

"The suggestion that unless we can prove a link, it's considered safe rather than showing a degree of safety as we would for other significant exposures such as medications is always of concern, said neurologist Dr Kelly Bertram, president of the Movement Disorder Society of Australia and New Zealand.

"Other jurisdictions around the world have seen the same level of data and have made a very different decision, which is not just to restrict the use but actually ban the sale of paraquat in the country," Dr Bertram said.

The APVMA did not answer specific questions sent by the ABC, but in a statement said it acknowledged the "concerns of an association between paraquat exposure and Parkinson's disease," and that health and safety "is the central consideration".

The regulator has confirmed to the ABC it has entered into a nine-month contract costing $109,000 with GRACosway, a public relations company that describes itself as a specialising in crisis communications.

"As the independent regulator, the APVMA must follow a rigorous statutory process and is unable to pre-empt or release the findings of final decisions before they are formally published," the APVMA said in a statement.

A spokesperson for Assistant Minister for Agriculture Anthony Chisholm said "the APVMA is using the full suite of its regulatory powers to promptly finalise long-standing chemical reviews".

"Australians can have confidence that APVMA decisions are based on the best available science to improve health and safety outcomes for people, animals, and the environment," they said.

Precautionary principle

According to the World Health Organization's (WHO) Global Burden of Disease study, Parkinson's disease is "the fastest-growing neurological disorder in the world in terms of disability and death".

"We know that maybe 10 to 15 per cent of [Parkinson's disease] cases have a genetic cause, but what about the other 80, 85 per cent?" professor David Blacker said.

Dr Blacker is not only a neurologist and movement disorder specialist, but he also has Parkinson's disease himself.

He said consideration needed to be given to "the precautionary principle".

"When you're involved with an activity that raises the risk of harm to humans or the environment, precautions can be taken even if you don't have a fully developed cause and effect relationship established.”

Dr Blacker also made a submission to the APVMA and has received no follow-up.

Farm groups say paraquat is 'essential'

Despite the long-running health concerns, the agriculture industry has come to rely on paraquat for weed control.

Some weeds are resistant to glyphosate, also known as Roundup, and ploughing paddocks can disturb soil structure and moisture.

"Without viable or effective access to paraquat and diquat products, farmers may be forced to return to traditional tillage to manage weeds and resistance," the National Farmers' Federation (NFF) said in its submission to the APVMA review.

NFF president Hamish McIntyre, himself a broadacre farmer, said paraquat was "an essential".

"Products like glyphosate and paraquat have really enabled us to change our farming systems in Australia and we farm on a drier continent than many of our competitors and so every bit of a moisture is critical to us."

According to McIntyre, there isn't a like-for-like replacement for paraquat and other options require waiting longer to re-enter a paddock after spraying.

"We do have our phenoxy type herbicides also, but there are plant-back issues with those," he said.

"We'll be guided by our independent regulator and if they say it's appropriate to pull back in certain chemistry, we'll follow their lead."

Paraquat 'volatilisation'

The international context has also shifted. In November 2025, the US Environmental Protection Agency (EPA) said a new vapour pressure study had increased uncertainty about paraquat's potential to volatilise, or move into the air as vapour, potentially increasing risks for workers and bystanders.

"The new vapor pressure study indicated a higher vapor pressure value for paraquat than was previously used in EPA's risk assessments," the US EPA said.

"Vapor pressure is a key parameter influencing the extent to which pesticide surface residues may convert into gaseous vapors that could move through the air, which could potentially impact workers and bystanders who live or work near or adjacent to treated fields."

The agency did not make any conclusions, and its review is ongoing.

The APVMA has since said it too is "reviewing its assessments of the volatility of paraquat" before it finalises its regulatory decision.

"This additional time will allow consideration of the updated review and, if necessary, refinements to the APVMA's risk assessments," the authority said late last year.

As of 2024 the APVMA's latest technical report states "provided conditions of registration and label instructions are followed," paraquat "would not be likely to have an effect that is harmful to human beings".

The regulator did not answer the ABC's questions about whether this was still its position following the US EPA's review of the new vapour pressure data.

First US state bans paraquat

Last week Vermont became the first US state to ban paraquat, marking a significant departure from federal policy.

Like Australia, the US Environmental Protection Agency (EPA) continues to allow restricted use of the pesticide despite ongoing health concerns.

Neurologist David Blacker recently attended the World Parkinson Congress in US, and has been following the Vermont ban closely.

"In anticipation of that, at least a dozen states have been particularly ready and poised to introduce legislation to ban paraquat," Dr Blacker said.

"I know that California for several years has been looking to introduce a phased out scheme … about a dozen states in total, but Vermont was first off the block."

What is epidemiology?

At the heart of the tension between clinicians and the regulator is the weight the regulator gives to epidemiology, or research that looks at patterns of disease in human populations rather than laboratory experiments.

While it is not possible to experiment with paraquat on humans, Dr Thevathasan said real-world exposure data could be just as compelling.

"Multiple epidemiological studies from different groups of scientists have shown that people exposed to paraquat have around a three times greater risk [of developing Parkinson’s disease]."

Dr Thevathasan said the risk was greater with chronic exposure and that there was a relationship with dose.

"We're probably never going to get anything ever better than that," he said.

"It's the same way we proved or showed that smoking caused lung cancer, that we showed that asbestos causes lung disease and lung cancer, this is the same methodology."

The APVMA has previously stated its analysis of studies and research might result in "different conclusions to study authors".

"To inform the paraquat proposed regulatory decision, the APVMA considered a wide range of studies relating to paraquat, including animal and epidemiological studies, as well as the conclusions reached by the US EPA," it said.