http://www.camh.net/pdf/lsd_dyk.pdf
The use of LSD can result in long-term effects in one-time and regular users of the drug. Possible negative effects are "flashbacks" of the drug experience, as well as prolonged anxiety, depression or psychosis. These reactions usually decrease over time, and end within a few months after LSD was last taken,
but may continue for years.
"Flashbacks" are the spontaneous and unpredictable replay of an aspect of the LSD trip, occurring some time after the initial effects of the drug have worn off. Visual or emotional experiences that were originally seen or felt while under the influence of LSD are re-experienced. Flashbacks usually last only a few seconds or minutes, but may happen over and over again. Only some people who take LSD have flashbacks, but frequent users of the drug are said to be at greater risk. Flashbacks may be triggered by smoking marijuana or drinking alcohol, or by emotional stress, fatigue or meditation.
Depression or anxiety may follow a "bad trip." Psychosis may develop after using LSD, although it is thought that this reaction may be more likely to occur in people with latent or underlying mental health problems.