Mr. Monk and the Miracle - S7-E9
Corrected entry: The pharmacist's plan is really stupid. He changes all their meds so they don't work, after drinking from the fountain he changed them back so it appears the fountain healed them. The problem is if they think the fountain healed them they're not going to take their meds, so their pain would come back.
Correction: He didn't just change their medication so it didn't work, he changed it so they actually got sicker or worse. Even if they stopped taking their real medication, they'd still feel better than before when they were worse off and still think it was the fountain. But since he wasn't trying to scam anyone out of any money, he didn't need a long term solution for when their pain came back. Just enough to stop the monastery from digging up the fountain.
Bishop73