Cyanide is a potent poison not readily available in pure form to the general public, a medical doctor and a police toxicology expert said on Wednesday.
Forensic tests have shown that cyanide was the toxin chosen by the person who has threatened Pick 'n Pay to poison its products in an attempt to extort money.
Jeanne du Plessis of the Poisons Information Centre at Cape Town's Red Cross Children's Hospital said potassium cyanide and hydrocyanic acid were available to industries using the substances.
One third of a teaspoon of potassium cyanide could kill an adult. One teaspoon of hydrocyanic acid would be lethal.
The head of the South African Police Service's forensic science laboratory in Pretoria, Superintendent Alida Grove, said sodium cyanide was also available.
She could not say which had been used by the Pick 'n Pay extortionist.
On Tuesday police and Pick 'n Pay revealed in a joint statement that a blood test had shown that the woman who ate a sardine from a tin bought in a West Rand branch of Pick 'n Pay and allegedly doctored by an extortionist, had been poisoned with cyanide.
"The process we use shows that cyanide was used, but it does not show which type," she said.
Du Plessis said it was impossible to say how low a cyanide level would not cause any side effects. Each person was different. "You can't line people up and feed them cyanide to see what happens," she said.
Du Plessis and Grove said the general public would not be able to easily get hold of cyanide.
"You would have to work in a place that uses it," said Grove.
Cyanide is used in specialised manufacturing, mining and laboratories.
The police and Pick 'n Pay said the deadly poison was also used in a tin of pilchards which the company found in a Boksburg branch after being told of its location by the extortionist.
Cyanide was also used - in far higher levels - in a parcel the extortionist sent to Pick 'n Pay in May containing three threatened products.