Experimenting with sugar and alternative medicine

In Terry Pratchett's books the witch Granny Weatherwax uses Headology to help people more than she uses magic. Headology uses both psychology and trickery to adjust people's behaviors. In the case of a man with chest pains, Granny told him that he'd been bewitched by nymphs who lived in a waterfall. All he had to do was hike to that waterfall, bow three times, sing a song, and leave a small offering once per day and the nymphs would leave him alone. Or something like that. My memory of t...