I like the acid trip theory myself, seeing as how LSD is can be formed by the action of a common fungus on ordinary grain. During the Middle Ages, there were stories of entire villages going "mad" when the local grain supply at the mill became contaminated.
...or maybe John enjoyed really earthy 'shrooms?
"I wish to see, and I mean this most sincerely, I wish to see the last King strangled with the guts of the last Priest." - Voltaire