I have read that marijuana was widespread throughout the Old World by the Dark Ages. Why isn't cannabis use a more common trope about the middle ages, the way that mead, ale and beer are?
Short answer: No. Cannabis is indigenous to south Asia, whereas smoking was a practice first developed in the Americas. Trans-Atlantic travelers in the 1500s would smoke tobacco when they were in the Americas, but most Europeans looked down on smoking as something backwards or dangerous. Only in the 1600s did Europeans begin to import and smoke tobacco in large quantities. Tobacco smoking probably reached the Ottoman Empire (Turkey and the Middle East) around 1610, and the Ottomans soon began to experiment smoking other substances as well. Marijuana smoking began shortly thereafter, and the water pipe (hookah) was soon developed to further facilitate this practice.
But there's reason to believe that cannabis was at least occasionally used for its psychoactive effects well before the modern period. Cannabis shows up in the archaeological record beginning around 3000 BC. Herodotus wrote in the 400s BC that the Scythians (a Central Asian steppe people, around the Black and Caspian Seas) would make incense from the seeds of a hemp plant, which might have been cannabis. Archaeologists found a Scythian grave from that same time which included a tent, a censor, and cannabis seeds. (Herodotus was often inventive, so corroborating evidence is useful.) As far as I know, however, this is an isolated find, which means that using cannabis in incense was probably not a wide-spread practice and may have been very short-lived; it might only have been coincidence that Herodotus found out about this particularly interesting Scythian fad.
Consistent reports of using cannabis for its psychoactive properties only begin to appear in the 1100s in Arabic sources (at the height of the spread of Sufism). These make it clear that the cannabis was being chewed or eaten to produce an intoxicating state. But as is often the case with these sorts of texts, we can't be 100% sure that the plant being talked about is the same one that we call cannabis today. Language can change rapidly. After all, in the 1100s English sounded something like this).
That's not to say that there weren't other psychoactive substances available in medieval Europe. For instance, several hundred seeds of henbane showed up in one Viking Age female grave in Denmark, and henbane later appears as a common ingredient in witches' ointments for shape shifting. This isn't my area of expertise, so I'd recommend posting a question about psychoactive substances in the middle ages if you want other examples.
Marcy Norton, Sacred Gifts, Profane Pleasures: A History of Tobacco and Chocolate in the Atlantic World (Ithaca: Cornell University Press, 2008).
Neil S. Price, The Viking Way: Religion and War in Late Iron Age Scandinavia (Uppsala: Uppsala University, 2002), 205-6 ("Narcotics and intoxicants"). Price also notes that Viking Age sites have produced a total of five cannabis seeds, excavated with the Oseberg ship burial.
Gabriel Nahas, "Hashish in Islam, 9th to 18th Century," Bulletin of the New York Academy of Medicine 58.9 (1982), pp. 814-31.
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