A belief that they are being poisoned is a common delusion among psychotics.
http://www.google.com/search?q=psychotic+poisoned+delusion&sourceid=opera&num=75&ie=utf-8&oe=utf-8
Diagnosing political prisoners as psychotics was a common Stalinist show trial tactic.
Milosevic Given Wrong Medicine
UN war crimes tribunal denies request for Milosevic autopsy in Moscow
Even the court admits he was given the "wrong medicine." The only question is whether it was a) accidental; b) criminal negligence; or c) deliberate murder.
This persecution was becoming an embarrassment, because what was meant to be a great show trial to demonstrate the power of the court turned into a pathetic farce.
Soviet prison psychiatrists called it sluggish (asymptomatic) schizophrenia. Now the Bolshevik mindset is alive in the West.