A psychological phenomenon in which an individual experiences an episode of some of the subjective effects of LSD (this may be a positive or negative experience) long after the drug has been consumed and worn off -- sometimes weeks or months afterward. Despite anecdotal claims that flashbacks are common, studies have shown that these experiences are relatively rare.
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
I'm having a flashback. "No you're not!". That's Art Linkletter hype.