Peter Hainsworth, in his TLS review of Barbara Renolds' Dante: The Poet, the Political Thinker, the Man, notes that Reynolds suggests that Dante may have enjoyed cannabis (this would, of course, put him in line with many a modern hip-hop versifier):
But the novelties come thick and fast, beginning (so far as I was concerned) with the suggestion on page 10 that Dante and other poets he associated with in Florence as a young man might have given their visionary and dreamlike imaginings a boost with the stimulus of love-potions. These herbal stimulants, cannabis perhaps, may, it turns out later, be what Dante is referring to in the comparison, near the start of Paradiso, between his own “trans-human” experience and what Glaucus felt “on tasting of the herb” (nel gustar dell’erba) which made him into a sea-god. As Reynolds explains at greater length when she comes to the final vision of the Godhead, mystics did often use drugs of one kind or another in conjunction with fasting and meditation in their pursuit of visionary illumination. There is no reason, she argues, why Dante should not have done so too.
This was news to me. I'd never heard or read that medieval mystics were fond of these, ahem, stimulants. Does anyone have any additional information on this topic?