Debate continues over the nature and causes of flashbacks. Some say flashbacks are a manifestation of post-traumatic stress disorder, not directly related to LSD's mechanism, and varying according to the susceptibility of the individual to the disorder. Many emotionally intense experiences can lead to flashbacks when a person is reminded acutely of the original experience.
Several urban legends claim that flashbacks are the result of trace amounts of LSD or related chemicals being dislodged and released into the body after having been crystallized and stored in fat or spinal fluid cells. However, scientific research has disproven this conjecture; LSD is metabolized in the liver, as with many other drugs, and its metabolites are excreted normally in the urine. Drug studies have not confirmed that LSD can cause flashbacks or lasting psychoses
"Flashbacks are the third deadliest killer in animes; most of time someone dies, it's during a flashback."