Six men were being questioned tonight by police in connection with the discovery of traces of the toxin ricin at a flat in North London. Dr Thomas Stuttaford, left, explains how deadly it is.

What is ricin?

It is a poison made from the seeds of the castor bean plant. The leptins - the chemical term for the extract - are taken from the seeds and made into oil that is highly toxic. The ricin is left behind when castor seeds are pressed and made into castor oil, which is of course very good for you. The residue that is made into ricin is normally killed off in the manufacturing process by placing it in boiling water.

How poisonous is ricin?

Highly poisonous. It is in a completely different category to other more common forms of poison such as arsenic and prussic acid. I would say that it is one of the most lethal toxic poisons available. Only a tiny drop is needed to kill someone - just a microgram per kilogram of body weight for an adult if injected or three micrograms/kg if inhaled. Even chewing the castor seeds themselves is dangerous; about 20 would be enough to kill an adult and as few as five could kill a child. But exposure to the skin is not lethal in any dose.

How effective would it be as a terror weapon?

I don't think it would be that effective as a weapon for waging mass terror. It does not have that capability. It could, however, be used to create alarm and despondency among the ruling classes, as prominent people die inexplicably. All they would feel is a light pinprick and soon they would be dead. Everyone would be asking: "Who is next?" It would create panic of a different order than say anthrax or smallpox.

How can ricin be administered?

Its bes-known use was in the killing of the Bulgarian dissident Georgi Markov in London in 1978, when he was stabbed in the thigh by an umbrella with a ricin pellet hidden inside the engineered tip. The dose was so small – the size of a pinhead – that the doctors carrying out a post mortem spotted the pellet only after taking an X-ray.

It could also be used in crystal form and placed in food for example, though a lethal dose would need to be much higher, somewhere in the region of 30 micrograms/kg. It could also be administered in aerosol form, which would require an even larger dose.

What are the effects of ricin?

Within minutes the victim will experience conditions similar to gastroenteritis. Initially the victim would suffer a high temperature and diarrhoea. Haemorrhaging and a breakdown of the red blood cells would follow, as would a failure of the kidneys and liver. The victim would then experience convulsions and within a matter of hours of the dose being administered he or she would be dead.

Is there any antidote to ricin?

No. One should pump the stomach as soon as possible; the patient would then have to be treated symptomatically as the poison takes hold. Death is certain.

What is ricin used for by the medical profession?

It used to be popular in the treatment of cancer. Tiny doses of ricin were delivered to the area of the body that is affected by the tumour by mono-clonal antibodies. It was not very effective and in the last ten years has been used less by the medical profession.

How easy is it to get ricin?

There are large amounts of stocks available throughout the world. Many people bring back bracelets and necklaces made of castor seeds from their holidays in the Third World, where the plant is widely grown. Anyone with a reasonably good knowledge of biochemistry would be able to make ricin in a laboratory.

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