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Flashbacks
There is also a commonly reported possibility of "flashbacks", 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. Flashbacks are an example of a Hallucinogen Persisting Perception Disorder, a DSM-IV diagnosis. Several journal articles have described the disorder (see, for example, Adverse consequences of lysergic acid diethylamide, H.D. Abraham and A. Aldridge, Addiction 1993, 88:1327-1334). Several studies have tried to determine how likely a "normal user" (that is a user not suffering from known psychiatric conditions) of LSD is to experience flashbacks. The larger studies include (Flashback phenomena in basic trainees who enter the US Air Force, Blumenfield, Military Medicine, 136, 39-41, 1971) and (LSD Flashbacks and Ego Functioning, Naditch, M & Fenwick, Journal of Abnormal Psychology, Vol. 86, No 4, 352-359, 1977), arriving at figures of 20% and 28%, respectively.
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 (which is water soluble) is metabolized in the liver, as with many other drugs, and its metabolites are excreted normally in the urine.
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Psychoses
There are some cases of LSD inducing or triggering a psychosis in people that were apparently healthy prior to taking LSD. In most cases, the psychosis is of short duration, but in other cases it may be chronic. It is difficult to determine if LSD in itself induces these psychoses or if it merely triggers latent conditions that would have manifested themselves otherwise. Several studies have tried to estimate the prevalence of LSD-induced prolonged psychosis arriving at numbers of around 4 in 1000 individuals.