re: Urban Myth? No, it is reality
Like marijuana, LSD attaches to the fat cells and if you are a bit of a chub will stay with you a relatively long time. In some heavy users it will react like a time release pill and the result is a flashback.
This is an additional negative to the possible "bad trip" with inharmonious company. Right now a lot of LSD is coming in letters from Eastern Europe and Florida clinics are seeing a fair amount of ongoing problems.
Publius