LSD, also known as acid, is a hallucinogenic drug that can be taken orally or absorbed through the skin. It causes visual hallucinations and alters a person's perception of reality. Short term effects include dilated pupils, changes in thoughts and feelings, dry mouth, and sweating. Long term effects may include flashbacks and the recurrence of trips. LSD was first synthesized in 1938 but its psychedelic properties were discovered in 1943. It is currently illegal in the United States and classified as a Schedule I drug.