In the case of Hawley Crippen, the unusual poison choice, Hyoscine, led investigators to question the validity of the remains. It seemed unusual for the toxicologist to check immediately for the common alkaloids before looking for more common poisons. But what are the more common poisons used in murders?
As far as the murder of individuals is concerned there are relatively few agents that have been used and of course there is no way of knowing how many people have been deliberately killed this way because no doubt many such deaths were attributed to natural causes. The following list is my estimation of the poisons most widely used in murders. Murder by poison is now extremely rare because modern forensic techniques make it possible to identify all toxic agents no matter now little remains in a corpse.
1. Arsenic: known in Roman times and used to poison rivals and even emperors. White arsenic, which is arsenic oxide, is a water-soluble, tasteless solid easily added to drinks. This material was obtained as a by-product of copper and lead refining. It the 1600s it was sold by agents of a woman known as Toffana of Sicily, to people who wished to dispose of someone and it became known as “inheritance powder”. In the 1800s arsenic compounds became widely available – as weed-killers, flypapers, rat poisons, etc. – and were used in domestic murders, being cited in many famous murder cases.
2. Atropine: aka “belladonna” and extracted from the juice of the berries of the deadly nightshade bush. In small doses this chemical causes hallucinations and was used for this purpose as long ago as ancient Greece. In larger doses it was reputed to be one of the favourite poisons of would-be murderers in Medieval Europe and the juice of only a few berries is fatal. The symptoms it produces would be easily mistaken for one of the many fevers which afflicted people in those days.
3. Strychnine: can be extracted from the seeds of the nux vomica tree, which grows in Southeast Asia and it became widely available in the west as trade with the Far East expanded. It was reputed to be a tonic and prescribed in small doses by doctors to aid convalescence. It was also widely used to poison rats and other animals and as such was easily obtained, and although cited in only a few domestic murders its ready availability suggests it would be used in many undiscovered murders.
4. Cyanide: can be distilled from the kernels of certain nuts such as almonds and also present in the leaves of some laurels bushes. The industrial chemical sodium cyanide is widely used, especially in mining, and has been involved in attempted mass murders. It was used to contaminate Tylenol capsules in the US in the 1980s and killed several people in the Chicago area. Cyanide has also featured in domestic murders and it causes death within minutes. It is the fastest acting of all poisons and for this reason it is the poison of suicide pills of the type carried by secret agents.
5. Thallium: this element was only discovered in the 1860s and while it has been used in some domestic murders – in some countries it has been available as rat poison – it has been more widely used as an agent of assassination. It is ideal in this respect. Thallium sulfate is water-soluble and tasteless and they take several days for the symptoms to appear and even then these are generally attributed to other illnesses. This poison was used by Saddam Hussein’s secret police and by the Russian KGB.
— John Emsley is author of Elements of Murder and Molecules of Murder.
https://www.pbs.org/wnet/secrets/the...p-poisons/202/
joe 
