When I was 17, I had a cold and the doctor prescribed some homeopathic remedy containing Atropine. Now, the whole point of homeopathy, of course, is to dilute any ingredients to the point where you are lucky to find even a single molecule of it in a given pill. (In my defense, I did not know anything about medicine back then.)

But I think I must have gotten a package from a batch where they forgot a couple of dilution steps or something. Also I may have misread the instructions. Anyway, it took me about six days to go through the pills, and during that whole time, I could not help but grin almost the entire time. Also, two or three times, I very clearly heard someone talking to me who was not there. I did recognize the voices, they belonged to people I knew. Then again, I looked around, did not see the person and figured that it must have been in my head, which apparently people hallucinating from Atropine are rather incapable of.

OTOH, I had none of the side effects Atropine has according to Wikipedia, so who knows what was going on with me back then. But I understood something was off, and the only thing that had changed from the weeks before were those homeopathic pills. (Just to be clear, all in all the experience was pleasant.)