Once commonly reported by medical facilities, acute adverse LSD reactions rarely are seen today. The paucity of users seeking emergency medical treatment might reflect increased knowledge of how to deal with such situations on the part of the drug-using community, as well as a decrease in the doses of LSD currently used compared with those used in the past. An individual's experience of the effects of the drug can be either pleasant or unpleasant; a perceptual distortion or illusion, while anxiety-provoking in one individual, can be pleasant for another. Social factors, media presentations, and public fear have all shaped perceptions of the drug's effects. Individuals who place a premium on self-control, planning, and impulse control tend to do particularly poorly on LSD. Prediction of who will have an adverse reaction is unreliable (Ungerleider et al. 1968). A history of positive LSD experiences renders no immunity from an adverse reaction. Traumatic and stressful external events can precipitate an adverse reaction (e.g., being arrested during a pleasant experience can precipitate an anxiety reaction). Although in general the hallucinogenic effects are proportional to dosage levels, adverse reactions have occurred after doses of LSD as low as 40 mg, whereas no effects have been reported from using 2,000 mg. Thus, acute adverse behavioral reactions generally are not dose related but are a function of personal predisposition, set, and/or setting.