Hallucinogens change the way you sense the world around you. LSD is odorless, colorless, and tasteless. It can be painted onto small squares of paper that people lick or swallow.

When you use LSD, your senses of space, distance, and time become altered. People say they "hear" colors or "see" sounds, but effects are unpredictable.

Once you go on an "acid trip," you can't get off till the drug's done with you — in about 12 hours! Strange feelings and strong emotions are typical. LSD can cause "bad trips" — users experience panic, confusion, sadness, and scary images.

Bad reactions can occur even with the first use and a user may have flashbacks, where a person experiences the feelings of a bad trip even after the drug wears off.

LSD affects judgment and behavior may get out of control. The user may find himself or herself in a dangerous situation.

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