Drugs, recreational and pharmaceutical, use of.
Medieval European is fairly simple. Alcohol, and not distilled stuff either -- there was some "winter wine" frozen outdoors, but the alchemists were still working out the details of distillation. (And boy was it a hard hit when gin actually became cheap enough to actually be drunk by the lower classes. There's a drug problem you never see in fiction.)
Then there's the Victorian era. Skipping lightly over the question of adulteration and straightforward counterfeits, there's the little question of what they actually could sell over the counter. There was one vegetable elixir -- very popular among the temperance women -- that turned out to hold a large proportion of alcohol. And you could buy infant soothing syrups, where the active ingredients were both opiates and alcohol, which soothed some infants right into an early grave.
Of course, in the early twentieth century the pure drugs began to take effect. Coca-cola thought it was better off for it, but then had a court case where it had to argue, at length, that they could not contaminate Coke with caffeine, it was one of the ingredients, advertised in the very name. . . .
There had been, in fact, a furor in Arabia when coffee was introduced, about whether it was banned as a mind-altering substance and therefore an intoxicant.
Even the history of the Opium War points to the recent introduction of opium into China. Such convolutions throughout history.