LSD stays in your spinal fluid forever and can trigger a trip years later.

The story was being passed around and argued about by 1999. On a Straight Dope Message Board thread about acid flashbacks, one poster blamed stories of trips set off by exercise for "the myth that LSD can be stored in your system" and "later released by heavy activities".[1]

Erowid's page on the myth describes the spinal version directly: people claim that "LSD has been found in spinal fluid years after the last time LSD was taken".[2]

LSD does not stay. In a 2017 study, healthy volunteers took 100 or 200 micrograms by mouth. "The plasma half-life was 2.6 h", and the drug showed "first-order elimination up to 12 h".[3]

Spinal fluid is not a store. A review of its physiology states that "CSF is renewed about four times every 24 hours".[4] A drug dissolved in it would be washed out, not kept.[4]

Nobody has shown LSD there years later. Erowid says of the claim: "There is no support for this claim." It asks to hear of any study that has "looked at spinal fluid of LSD users for LSD".[2]

Flashbacks are a separate and real thing. A 2003 review of 20 studies from 1955 to 2001 found that the word flashback "is defined in so many ways that it is essentially valueless". The persistent visual form, hallucinogen persisting perception disorder, "appears to be a genuine but uncommon disorder".[5] A 2014 review says its mechanisms "are largely unknown" and that "low-level visual processing may play an important role".[6] None of the proposed causes is a dose of LSD waiting in the spine.[2][6]