D-lysergic acid diethylamide (LSD) is the most potent hallucinogenic substance known to man. Dosages of LSD are measured in micrograms, or millionths of a gram. By comparison, dosages of cocaine and heroin are measured in milligrams, or thousandths of a gram. Compared to other hallucinogenic substances, LSD is 100 times more potent than psilocybin and psilocin and 4,000 times more potent than mescaline.

LSD is classified as a Schedule I drug in the Controlled Substances Act of 1970. As a Schedule I drug, LSD meets the following three criteria: it is deemed to have a high potential for abuse; it has no legitimate medical use in treatment; and, there is a lack of accepted safety for its use under medical supervision.

The effects of LSD are unpredictable. They depend on the amount taken, the user's personality, mood and expectations, and the surroundings in which the drug is used. Usually, the user feels the first effects of the drug 30-90 minutes after taking it. These effects include dilated pupils, higher body temperature, increased heart rate and blood pressure, sweating, loss of appetite, sleeplessness, dry mouth, and tremors. Sensations and feelings change much more dramatically than the physical signs. The user may feel several different emotions at once or swing rapidly from one emotion to another. Depending on the dose, the drug can produce delusions and visual hallucinations, which can be frightening and cause panic. Users refer to their experience with these acute adverse reactions as a "bad trip," and the effects typically last for about twelve hours. Terrifying thoughts and feelings, fear of insanity and death, injuries, and fatal accidents have occurred during states of LSD intoxication. Anyone can experience a bad trip and there is no way to predict what your own experience will be.