Paraquat is one of the world's most widely used herbicides. It's also highly toxic and some research points to a link with Parkinson's disease.
Its use is highly restricted in some countries, and banned in others .
Towards the middle of 2026, Australia's pesticides regulator is set to release its long-awaited regulatory review into the herbicide, after public consultation and considering the latest data from the United States .
Let's see what the evidence says about paraquat's toxicity and any link with Parkinson's disease.
What is paraquat?
Paraquat is a non-selective herbicide that basically kills all the vegetation it touches. It's rapidly absorbed by the leaves. There it generates unstable, oxygen-containing molecules known as "reactive oxygen species". These break down the leaves' cell membranes, destroy the plant's green parts and stop photosynthesis. Essentially, with photosynthesis gone, the whole plant dies.
In Australia, paraquat is used widely in broadacre cropping to kill weeds and is an alternative to the common herbicide glyphosate. It's also used for spot spraying. It's popular with farmers because it can be used on a variety of weeds. It's also rapidly inactivated when it touches the soil. This means it doesn't remain in the soil to affect future crops.
However, the generation of reactive oxygen species that kills weeds also makes it acutely toxic to humans. If it is inhaled, ingested or touches the skin/eyes it can cause symptoms from irritation and burns to failure of multiple organs and death.
Human toxicity and poisoning have led to many countries banning paraquat . For instance, the European Union banned it in 2007. China stopped all domestic sales and use of paraquat in 2016.
In the US, only trained, certified applicators can use it.
In Australia, paraquat is registered as a Schedule 7 poison . This means it's a dangerous poison with a high potential for causing harm at low levels of exposure.
Its purchase, possession, and use require special authorisation. It's formulated to give a distinctive blue or green colour, and unappealing smell. It cannot be used in home gardens.
Why has paraquat been linked to Parkinson's?
Paraquat may also be involved in developing Parkinson's. This is a progressive movement disorder . Over time, people develop rigid muscles, become less-able to move, have tremors, and become unstable on their feet.
There is no single known cause. But risk factors include increasing age, and a variety of lifestyle factors and environmental exposures. Air pollution and exposure to pesticides (including herbicides) are among those environmental exposures.
Paraquat is structurally similar to a molecule know as MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). When taken up by nerves containing dopamine, MPTP can cause Parkinson-like lesions in the brain.
As paraquat damages cells, through a process known as oxidative stress, it's possible it could damage nerves containing dopamine. But because paraquat cannot cross the blood-brain barrier via the dopamine transporter to enter the brain, it's unclear how it could cause damage there.
How strong is the evidence?
Despite paraquat's chemical similarity to MPTP, well-conducted animal studies with paraquat have not produced consistent results about any increased risk of Parkinson's.
Researchers have questioned whether the route of administration of paraquat in some animal studies (via injection through the abdominal wall) applies to humans . Humans are exposed typically by skin contact, inhalation and to a minor extent by accidental ingestion. All these routes involve less absorption of paraquat than injection, and more breakdown. Studies involving mice drinking paraquat had conflicting results .
The epidemiological evidence from humans exposed in either industrial or agricultural settings also provides no clear evidence of an increased risk of Parkinson's.
For example, one study looked back at people who had worked in paraquat production facilities. Even in people with moderate to high levels of exposure to paraquat, there was no evidence of an increased risk of Parkinson's.
Several studies have linked pesticide use in general to Parkinson's, but pinning that down to one particular pesticide or herbicide has been more difficult .
Some individual studies have indicated a link between paraquat and Parkinson's. But when you look at the entire body of evidence in a type of study known as a meta-analysis , we cannot say one definitively causes the other.
A more recent meta-analysis, looking at a broad range of epidemiological studies, concluded pretty much the same thing .
So, what's the take-home message?
The weight of evidence from animal experiments and human epidemiological data suggest it's unlikely there's an increased risk of Parkinson's in farmers and agriculture workers exposed to paraquat. There is no increased risk to the general public.
And on the evidence to date, while paraquat is highly toxic to humans, it does not cause Parkinson's disease.
Ian Musgrave has received funding from the National Health and Medical Research Council to study adverse reactions to herbal medicines. He has previously been funded by the Australian Research Council to study potential natural product treatments for Alzheimer's disease. He is currently a member of one of the Therapeutic Goods Administration's statutory councils
