Some fatal accidents have occurred during states of LSD intoxication. Many LSD users experience flashbacks, recurrence of certain aspects of a person’s experience, without the user having taken the drug again. A flashback occurs suddenly, often without warning, and may occur within a few days or more than a year after LSD use. Flashbacks usually occur in people who use hallucinogens chronically or have an underlying personality problem; however, otherwise healthy people who use LSD occasionally may also have flashbacks. Bad trips and flashbacks are only part of the risks of LSD use.
Environmental factors such as the availability of marijuana, expectations about how the drug would affect them, the influence of friends and social contacts, and other factors that differentiate identical twins’ experiences also were found to have an important effect; however, it also was discovered that the twins’ shared or family environment before age 18 had no detectable influence on their response to marijuana.