In the sixteenth century a Swiss physician and alchemist by the name of Paracelsus claimed that everything was potentially poisonous, as long as you took enough of it: ‘the right dose differentiates a poison and a remedy.’ There is plenty of evidence to support this point of view. Legal claims for damages caused by asbestos and passive smoking are reminders that what may be a safe environment for some can be toxic for others. Indeed, one of the most common forms of contemporary poisoning is known as an ‘overdose’. The substance was fine. The amount was wrong.

But some substances are surely more lethal than others. And so are some people. Reading Gail Bell’s account of scores of poisonings found both in literature and medical history, you begin to wonder if it is simply circumstances that distinguish a poisoner from the healer.