Among many legends surrounding drug use, this one seems the most ridiculous! As the myth would have it, acid users could be at risk of a “surprise” hit years after they ingested the drug. Supposedly acid would crystallise in the spine or in fat tissues, dislodge itself years later and create a horrifying flash back effect. It is true that the body does store toxins in its fat tissue and that it is possible to find traces of some drugs within this tissue or the spine. However, LSD has a half-life (time it takes for it to degrade) of 3-5 hours and by its chemical composition, is insoluble in fats. Its residues are never found in the body.
It is possible the myth is born from the fact that heavy use can result in psychosis and a persisting halucogenic disorder. We now know this is due to psychology, not chemical compounds.