During both World Wars, Agatha Christie worked in a dispensary--a medical office that filled prescriptions, similar to a pharmacy. The knowledge she gained there of poisons served her well in her writing career.
Regarding her experiences in the dispensary during World War I, she wrote:
Later that year [probably 1914] I had flu badly, and after it congestion of the lungs which rendered me unable to go back to the hospital [to work as a nurse] for three weeks or a month. When I did go back a new department had been opened--the dispensary--and it was suggested that I might work there. It was to be my home from home for the next two years. . . . It was while I was working in the dispensary that I first conceived the idea of writing a detective story . . . and my present work seemed to offer a favourable opportunity. . . . Sometimes I would be on duty alone in the afternoon with hardly anything to do but sit about. Having seen that the stock bottles were full and attended to, one was at liberty to do anything one pleased except leave the dispensary. I began considering what kind of a detective story I could write. Since I was surrounded by poisons, perhaps it was natural that death by poisoning should be the method I selected. I settled on one fact which seemed to me to have possibilities. I toyed with the idea, liked it, and finally accepted it. Then I went on to the dramatis personae. Who should be poisoned? Who would poison him or her? When? Where? How? Why? And all the rest of it (An Autobiography, 235, 241-2).