June Cronin-Simpson, 62, showed no emotion when she was sentenced at Bristol Crown Court by Judge Peter Thomas, who described her as a "potentially very dangerous lady".
Cronin-Simpson, who was appearing for sentence after being convicted in early November on three charges of causing a noxious substance to be administered, had refused to co-operate in a psychiatric assessment.
The court was told that Cronin-Simpson, of Yeovil, Somerset, drilled holes in the neighbour's walls and inserted piping to pour a mixture of petrol and chemicals into the house next-door.
The judge, who jailed Cronin-Simpson for six years on each count, the sentences to run concurrently, said that what she had done was "exceptionally dangerous".
He had no alternative but to jail her because she refused the psychiatric assessment.
Judge Thomas said: "Because of her failure to co-operate with the medical assessments we had no option but to impose an immediate prison sentence."
He said it was fortunate that no ignition had taken place in the home of Julian Geard, his wife Joyce and their 18-year-old daughter Rosemary, who were living next door to Cronin-Simpson's elderly mother, also in Yeovil, at the time of the attack.
The poison attacks on her mother's neighbours were apparently motiveless, as the Geards scarcely knew Cronin-Simpson. The family was mystified by the incidents.
The court was told that Mr Geard had noticed a mixture dribbling down the walls of his bathroom in December 1997.
He later went into his loft to find that four smoke bombs - designed to kill moles and rats - had been put in the roof space, causing charring of insulation materials.
Cronin-Simpson was ar-rested two days later and sectioned for some weeks under the Mental Health Act.
Malcolm Galloway, for the defence, said in mitigation that a psychiatrist who examined Cronin-Simpson in October could find no evidence of mental disorder. While on remand in prison she had proved a model prisoner, he said, and fortunately no one was seriously injured by her attacks. Cronin- Simpson's 85-year-old mother is now in care.