A single teaspoon of botulinum toxin could kill seven million people, but even tinier doses of the poison can be used to ease the suffering of patients with dystonia, a condition which causes uncontrollable spasms.
As a weapon, the toxin is designed to paralyse the respiratory system, leading to death by suffocation. But as a medicine it can be used to paralyse muscles and prevent them from responding to involuntary signals from the brain.
Dystonia can leave patients with their eyelids constantly twitching and blinking. Sometimes their eyes will not open at all and have to be prised apart with their fingers. Others suffer from a form of dystonia called torticollis which forces them to go into spasms and involuntarily twist their neck to the side.
According to Alan Leng, the chief executive of the Dystonia Society, botulinum toxin is a crucial aid to allowing such patients to live a more normal life. But many health authorities consider that at around pounds 270 a vial it is too much of a strain on their budgets.
"Hospitals are not providing the doses that are required because the health authorities are saying they don't have the money," Mr Leng said.
"People have been casting around for years for treatment and they are often desperate. This [toxin] brings a lot of relief to a lot of people."
There are 38,000 dystonia sufferers in Britain but the condition often goes unrecognised by doctors and by the authorities.
John Orford used to stand on duty outside Buckingham Palace as a Grenadier Guardsman until he was struck down by torticollis and was no longer able to stand up straight.
Unable to stop his head from twisting to the side he was mercilessly taunted by junior officers who said he resembled a nodding dog in the rear window of a car.
"The muscles seem to have a will of their own," he said. "I start shaking and my head will turn to the right, but if you try and fight it it only makes things worse."
Mr Orford believes he was in perfect medical condition until he wrenched his neck while doing sit-ups on an inclined bench as part of his army fitness training at the age of 18.
The incident triggered his torticollis and from then on he was unable to hold himself erect while on guard duty at the Palace or outside the Tower of London. Army doctors said he had "wry neck" which they later told him had "settled".
Mr Orford continued to serve in the Guards, albeit on minor duties, and despite being taunted by colleagues to the point where he said he became "suicidal".
An Army psychiatrist told him that he was suffering from "phobic notions". Then, in 1979, four-and-a-half years after he joined the regiment at the age of 17, he was made to leave.
Weeks earlier, while on leave, his own GP had diagnosed torticollis. But instead of being given a medical discharge, the words "services no longer required" were written onto Mr Orford's record.
Since then his life, along with his medical condition, has deteriorated. He has been unable to hold down a job and now feels so ashamed of his shaking and twitching that he stays at home rather than risk being mocked at the pub or in shops. Only the botulinum toxin jabs give him temporary relief.
For nearly two decades he has campaigned for his record to be changed "to clear my name" and to show he could have been a competent guardsman if it had not been for his condition. But despite support for his claims by some of Britain's leading neurologists, the Ministry of Defence insists that the decisions of Army Medical Boards are final.
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