I’ve never done any, but I used to work with someone who said he used to work with someone who had been a heavy acid user back in the day, but stopped.

The intermediary said that the original user would occasionally stretch, popping his vertebrae, then have to take a brief timeout as he’d released essentially a microdose.

I don’t know enough about drugs to know if that’s plausible (and haven’t thought about this anecdote in years) but the idea of having an unexpected trip at any moment always sounded very worrying.