LSD can temporarily impair judgment and increase risk of accidents. Users may experience "flashbacks" where they re-experience effects of LSD days after use, though these are generally short-lived and mild. Albert Hofmann first synthesized LSD in 1938 while researching medical applications. LSD has unpredictable short-term effects like dilated pupils and altered sensations, but can cause hallucinations at high doses. Long-term effects may include flashbacks or developing psychoses like schizophrenia. While not addictive, users can build a tolerance requiring higher doses for effects. LSD is used by teenagers and young adults, especially at concerts and raves.