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| LSD |  The mechanism by which LSD alters perceptions is still unclear. LSD is a hallucinogenic drug, that acts on certain groups of serotonin receptors designated the 5-HT2 receptors and that its effects are most prominent in two brain regions: one is the cerebral cortex, an area involved in mood, cognition, and perception; the other is the locus ceruleus, which receives sensory signals from all areas of the body and has been described as the brain's "novelty detector" for important external stimuli.
LSD's effects begin from 30 to 90 minutes after ingestion and may last for 12 hours. Users consider to LSD and other hallucinogenic experiences as "trips" and to the acute adverse experiences as "bad trips." LSD trips usually include both pleasant and unpleasant aspects, the drug's effects are unpredictable and may vary with the amount ingested and the user's personality, mood, expectations and surroundings.
Peope, which use LSD may experience some physiological effects, such as increased blood pressure and heart rate, dizziness, loss of appetite, dry mouth, sweating, nausea, numbness, and tremors; but the drug's major effects are emotional and sensory. The user's emotions may shift rapidly through a range from fear to euphoria, with transitions so rapid that the user may seem to experience several emotions simultaneously.
LSD has bad effects on the sensations. Sounds, colors, smells and other sensations seem highly intensified. In some cases, sensory perceptions may blend in a phenomenon known as synesthesia in which a person seems to hear or feel colors and see sounds.
Hallucinations may distort movements and shapes. They activate a perception that time is moving very slowly or that the body of the one, who uses LSD is changing shape. On some trips, users experience sensations that are enjoyable and mentally stimulating and that produce a sense of heightened understanding. Bad trips, however, include terrifying thoughts and nightmarish feelings of anxiety and despair that include fears of insanity, death or losing control.
LSD users quickly develop a high degree of a habit for the drug's effects. After using this drug rapidly they need increasingly larger doses to fill the similar effects. LSD use also produces tolerance for other hallucinogenic drugs such as mescaline and psilocybin, but not to drugs such as marijuana, amphetamines and PCP, which do not act directly on the serotonin receptors affected by LSD. A habit for LSD is short-lived it is lost if the user stops taking the drug for several days. There is no evidence that LSD produces physical withdrawal symptoms when chronic use is stopped.
Two long-term effects, persistent psychosis and hallucinogen persisting perception disorder (HPPD), more commonly referred to as "flashbacks", have been associated with use of LSD. The causes of these effects, which in some users occur after a single experience with the drug are not known.
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