system
Given the following post, respond as if you're commenting on Reddit:
human
My friends have always told me that LSD hangs around in your spine for years after you use it, which I always assumed was a fairy tale. However, I'm curious about the actual effects of LSD and why users experience flashbacks. Can anyone elaborate?

BTW, I am not a user, nor have I ever used LSD. Just really fascinated by the brain. Thanks!

**EDIT:** Thank you guys so much, especially itsbrownmagic, for weighing in on this! Awesome information guys and girls!
gpt
Medically speaking, LSD is metabolized quickly (half life of 175 minutes). That being said, it can be concluded that even if LSD somehow crossed the blood-brain-barrier and entered your CSF (cerebrospinal fluid), it's metabolism is so rapid that it would not be there for any relevant period of time.

Leading theories on the phenomenon of "flashbacks" point to 2 possible major causes:

#1) The flashback is similar to PTSD (fear conditioning) where the user had a "bad trip" and certain cues later on in life can bring the person back to that trip. This would be facilitated by your memory circuits in the hippocampal region of the brain.

#2) Use of LSD inhibits and/or activates certain neurons, which when stimulated, can give the impression that one is "tripping" again. This isn't because the drug is still in your system, but is because the drug has modified receptors in the brain permanently. The region thought to be affected would be the lateral geniculate nucleus of the thalamus.