Safeway was forced to withdraw stocks from supermarket shelves across the country after the 'foreign substance' was found in bottles bought from its Hunter's Tryst store.
Initial tests suggest the poison was of the atropine type, which is found in deadly nightshade. Detectives have still not determined whether the tonic water had been poisoned deliberately.
Elizabeth Smith, 45, and her 18- year-old son, Andrew, from Edinburgh, were taken to hospital last Friday after drinking Safeway's own-brand tonic water. They were released after treatment.
Yesterday, it was disclosed that a second woman, Alexandra Agutter, 39, and her 11-year-old daughter, Beatrice, were also treated in hospital on Sunday after drinking tonic water brought at the Hunter's Tryst store.
Mrs Agutter was discharged from Edinburgh's Royal Hospital for Sick Children, but Beatrice remained under observation in the intensive care unit.
Safeway has removed all two-litre bottles of diet and regular tonic water with the batch codes 4208 and 4209 from its 371 stores. The bottles under suspicion had been tampered with externally.
Dr Geoffrey Smith, 48, an anaesthetist whose wife and son needed treatment on Friday, said he had traced the source of the poison to the tonic water through a process of elimination because both he and his 13-year-old son, Nicholas, had not drunk any of the tonic water. He also remembered his wife saying there had been no seal on the bottle.
Both his wife and son had drunk the tonic water on Wednesday evening, began complaining of dry mouths, and finally went to bed feeling very ill. Next morning his son complained of 'vivid dreams'. On Friday, Dr Smith returned from duty at 12.15am, to find his wife and son again very ill.
Detective Superintendent John McGowan, of Lothian and Borders Police, said that officers had yet to establish how the poison had got into the tonic water or a motive for the act.Reuse content