jadeius:
but an lsd flashback is not something tied to a memory...it is a chemically-induced reoccurance of a drug-related experience attributed to the buildup of the particular drug at a particular site and the release of that particular chemical at a particular time based on particular stresses on that site...that is, if a flashback even really exists
Erowid:
There is an often circulated myth that once you have taken LSD, it remains in your body forever. The main thing that keeps these rumors circulating the is fact that some people (though very few) experience "flashbacks" (generally within a few months after a hallucinogenic experience). It is generally accepted, however, that these flashbacks are not the result of lsd remaining in the system.


http://www.erowid.org/chemicals/lsd/lsd_myth1.shtml

http://www.erowid.org/chemicals/lsd/lsd_health2.shtml