A pensioner living in a US care home has admitted making ricin and testing the deadly toxin on her neighbours by slipping it into their food and drink.

Betty Miller, 70, told FBI agents she exposed other residents to ricin at least three times after making three tablespoons of the poison from 30-40 castor beans she picked from plants at Wake Robin retirement home in Shelburne, Vermont.

She said she wanted to "injure herself" and had placed the ricin in her neighbours' food and drink to test its potency, according to court documents.

Just 1.78mg of ricin can kill an adult and there is no known antidote to the highly toxic protein. If ingested, symptoms include vomiting, diarrhoea, hallucinations and seizures.

US officials said they believe one person at Wake Robin retirement home became sick with ricin poisoning, but said no one was currently ill.


Image: Castor seeds

Police were called to the care home after Miller told health care providers she had made the toxin.

During a search of the pensioner's apartment, FBI Weapons of Mass Destruction investigators found a half-full pill bottle labelled "ricin", alongside other containers labelled "apple seed", ''cherry seed" and "yew seed", according to court documents.

The powder tested positive for ricin and instructions to make the protein were also found in Miller's apartment, the FBI said.

Miller was arrested and charged with unregistered possession of an agent.

She appeared at a federal court in Burlington, Vermont, on Friday.

During the hearing, judge John Conroy noted that Miller had a "lengthy mental health history", but did not elaborate.

In a statement, Wake Robin retirement home said Miller "is now involved with the criminal justice system and will not be returning".

It added: "The toxic substance was contained; no residents were evacuated.

"We have received assurances from the Vermont Department of Health and the FBI that no one's health is at risk."