LSD and magic mushrooms ("shrooms") are examples of hallucinogens. The experience of taking a hallucinogen is usually referred to as a "trip." In low doses, hallucinogens produce a spectrum of effects. These effects vary according to the particular drug and the individual user's sensitivity. Effects include alterations in mood and perception. More severe effects, such as hallucinations, are most likely to occur at high doses. The effects of any hallucinogen are different from person to person, and can range from ecstasy to terror. In fact, a person will often experience a wide range of reactions during the same trip.
People may experience different reactions to the same drug on different occasions, finding the effects at times pleasant and at other times disturbing and threatening. These unpredictable "bad trips" can occur even when the drugs are known to be pure and the same amount is used on different occasions.