Question here, even though I don't intend to do it: I recall having heard that, if you do LSD, you can have a small trip again unexpectedly even years later. That would scare the shit out of me, suppose it happens while you're driving.

Is that true at all? Or is it just another myth?

HPPD is definitely real, but I would describe that as a more or less constant experience of after-effects for weeks/months/etc.

I took enough LSD and other substances to experience HPPD that steadily faded over the course of about 3 months. Once it was over though, I never randomly experienced a small trip or anything.

From what I've read that idea comes from a myth that psychedelics build up in parts of the body (e.g. the spine) and can spontaneously be released. So, I wouldn't worry about it (and for what it's worth -- I'll get downvoted to hell for this -- but in the past I drove hundreds of times on psychedelics, without incident)

Have you ever smelled something that evoked a really vivid memory? Where every detail of a past experience was suddenly brought to the foreground in your memory? Maybe a certain kind of perfume reminds you of a past girlfriend or something like that.

In my experience, that's what people are talking about when they talk about "acid flashbacks". It's just that you tend to form very strong memories while tripping. It's not dangerous and it's not something unique to psychedelics.

They call them flashbacks and I suspect there's probably a large amount of urban legend going on. Anecdotally, I know many people who have taken psychedelics and have never heard a report of a flashback. That's not to say they're not a thing.

http://io9.gizmodo.com/5951972/could-you-actually-have-an-ls...