Some people who regularly use LSD may eventually experience flashbacks. This is when an LSD experience reoccurs usually a visual distortion that involves perceptual or emotional changes.Flashbacks can happen weeks, months or even years after the drug was last taken. Flashbacks can be disturbing, especially if a frightening experience or hallucination is recalled.1, 2Flashbacks can be brought on by stress, tiredness, exercise or using other drugs. They usually last for a minute or two.