LSD causes the senses of space, distance, and time to become altered. People say they "hear" colors or "see" sounds, and have strange feelings and strong emotions.

Many users refer to an "acid trip" — when the effects don't let up and can last for up to 12 hours at a time. LSD also can cause "bad trips" — when users experience panic, confusion, sadness, and scary images. Bad reactions can occur with the first use and a user may have flashbacks later, experiencing the feelings of a bad trip even after the drug wears off.

Because LSD also affects judgment and behavior, users might find themselves in a dangerous situation.

Physical changes include increased heart rate and blood pressure, muscle twitches and shaking, dilated pupils, sweating, sleeplessness, and loss of appetite.