[pmoriarty]

There are quite a few reports of people experiencing powerful somatic effects just from the anticipation of taking a drug, without having taken any.

Experiencing an unusual taste in the mouth is common. I've heard of some people feeling a need to vacate their bowels when they are anticipating taking a drug.

So it's not at all surprising that when you actually take a drug you experience something right away, even before the drug has had time to work.

The mind is a powerful thing.

This is a completely untested hypothesis, but I wonder if these kinds of effects are somehow related to hallucinogen persisting perception disorder (HPPD). People experience some really powerful effects under the influence, and maybe the mind somehow learns to reproduce some of these effects on its own.