I am not a historian.
The following links have convinced me that a subset of the European medieval population secretly indulged in hallucinogens.
I am very much open to correction.
HENBANE (Hyoscyamus niger) was often included in the witches' brews and other toxic preparations of medieval Europe to cause visual hallucinations and the sensation of flight. An annual or biennial native to Europe, it has long been valued in medicine as a sedative and an anodyne to induce sleep. The principal alkaloid of henbane is hyoscyamine, but the more hallucinogenic scopolamine is also present in significant amounts, along with several other alkaloids in smaller concentrations. Henbane is one of 20 species of Hyoscyamus, members of the nightshade family, Solanaceae. They are native to Europe, northern Africa, and western and central Asia.
https://www.erowid.org/library/books_online/golden_guide/g41-50.shtml
BELLADONNA (Atropa belladonna) is well known as a highly poisonous species capable of inducing various kinds of hallucinations. It entered into the folklore and mythology of virtually all European peoples, who feared its deadly power. It wos one of the ingredients of the truly hallucinogenic brews and ointments concocted by the so-called witches of medieval Europe.
http://soilandplants.blogspot.tw/2010_06_01_archive.html
Mandrake and other plants of the nightshade family (Solanaceae) contain alkaloids that block nerve impulses, which may lead to hallucinations. Although the cellular and molecular mechanism of action was only explained at the end of the 20th century, the pharmacological effects of these plants were already described by the Greco-Roman physicians Dioscurides (1st century AD) and Galenus (circa 129-199) and, from the 16th century onwards, by authors of herbal medicine books in local languages.
http://www.scienceinschool.org/2007/issue4/witchmedicine
Datura and related plants were used as hallucinogens by medieval Europeans.
http://www.scribd.com/doc/22245938/The-Role-of-Hallucinogenic-Plants-in-European-Witchcraft
Cannabis was known and used by educated persons, including alchemists, in medieval Europe.
Jabir Ibn el-Hayyan, known as Geber, gave instructions for the refinement of cannabis products.