By Jane Elliott
Health reporter, BBC News
Think of murder and the chances are your mind will spring to guns or knives, maybe even arsenic or strangling.
Insulin was hailed as a "wonder drug"
Few would consider the diabetes treatment insulin as a weapon, but over the last 90 years that is just what it has become in a few, often high-profile cases.
Insulin is needed by the body to stop the levels of sugar in the bloodstream getting too high.
People with diabetes cannot produce enough of the hormone or are unable to respond to it properly, and need more.
A murder weapon
But within just five years of its 1921 discovery, insulin was also being used to kill.
Too much of the hormone can reduce the amount of glucose in the blood to a level where the brain is unable to function properly.
The first recorded case of an attempted suicide using insulin was recorded in 1927, and by 1963 there had been a total of 13 cases.
Its first use as a murder weapon was recorded in 1957, and since then there have been about 50 cases globally of insulin being used either to kill, or to try to kill - mainly by medical staff who find the drug easier to obtain.
High profile cases
There have been a few very high profile cases, such as that of Claus Von Bulow in the US and the Beverley Allitt case in the UK.
Now Professor Vincent Marks, a world expert on insulin who has assisted in some of these trials, has written a book, Insulin Murders - True Life Crimes.
Beverly Allitt, a nurse, was given 13 life sentences for murdering and attacking children in her care by giving them insulin.
Allitt, dubbed the "Angel of Death" was detained at Rampton High Security Hospital, Nottinghamshire, in 1993 and was told she would serve a minimum tariff of 40 years.
In his book he talks about his experience during the Beverly Allitt "Angel of death" murders trial.
Nurse Allitt murdered at least four children and tried to kill another nine.
Prof Marks was asked to help establish that insulin had indeed been used and his expert witness statements were crucial in helping prove a case against Allitt.
But he said that first he had to convince the other medical experts at a specially convened conference that the deaths were not random.
William Archerd, a nurse, was found guilty in 1968 of murdering his nephew and two of his seven wives, but many suspect other killings.
In one of the murders Archerd claimed that robbers had come to his house armed with hypodermic syringes and injected both him and his third wife Zella, who later died.
"It appeared that almost no one except me was prepared at the beginning of the conference to stick their neck out and say foul play had definitely been committed," he said.
"I did this on the basis of baby Paul (Paul Crampton, aged five months - survived), who had been injected with a huge dose of insulin on three or more occasions."
Prof Marks also advised on the case of Von Bulow - a man twice tried for attempting to kill his socialite wife.
Claus Von Bulow
Claus Von Bulow was twice tried for attempting to murder his wife, Sunny - once in 1982 and then again in 1985.
The first time he was found guilty and sentenced to 30 years in jail, this was later quashed.
At the second trial he was acquitted.
He was once convicted and later, Prof Marks feels rightly, acquitted.
"The Von Bulow case highlights how a prosecution case that is totally without merit and that should never have been initiated in the first place can lead to an innocent person being convicted of a non-existent crime, merely for lack of expert knowledge and the difficulty of conveying complex scientific facts to a judge and jury," he said.
Von Bulow's wife fell into a coma, which the prosecution claimed had been triggered by a rogue insulin injection.
But Prof Marks argued that her condition was more likely to be related to heavy alcohol consumption.
Although insulin can be used to kill, Prof Marks said it was actually a very poor murder weapon.
Detecting its use was difficult, but not as many assumed, impossible.
"It is not a very good weapon, it is easy to kill babies and old people with insulin, but not adults."
Prof Marks co-wrote the book with journalist Caroline Richmond.