Lysergic Acid Diethylamide (LSD-25) is synthesized from lysergic acid, formed by ergot, a fungus that grows on rye. It was discovered by Swiss chemist Albert Hoffman in 1938, while he was working at Sandoz. He did not become aware of its effects until 1943, when he accidentally ingested some of the chemical.
During the 50s, the US Central Intelligence Agency (CIA) conducted a secret program known as MKULTRA, which investigated the usage of LSD for mind control and interrogation. 
LSD, during its brief history, has been used both spiritually and recreationally. Journalist Tom Wolfe, in his book The Electric Kool-Aid Acid Test, recorded the adventures of Ken Kesey's hippie enclave, the Merry Pranksters, who often held extravagant parties based around LSD consumption, typically consumed in spiked Kool-Aid.
Dr. Timothy Leary was an advocate of LSD use who believed that the chemical could be useful for psychology and spiritual pursuits. He created the catchphrase "Turn on, tune in, drop out" to encourage people to "tune in" to LSD, "turn on" to themselves and the world around them, and then "drop out" of the mainstream society, becoming self-reliant and enlightened.
Despite Leary's efforts to get LSD mainstream acceptance, the drug suffered from a lot of negative publicity. Many sensational urban legends sprung up around it. One was concerned with a young person who, while intoxicated on the drug, believed that they could fly, plummeting to their death out of a tall building. This tale appeared to come true in 1969, when Diane Linkletter, the daughter of famed TV show host Art Linkletter, committed suicide in this manner. Art claimed that she had been experimenting with LSD at the time, and went on a crusade against the drug. However, no evidence was uncovered to link her death to LSD usage.
In 1966, Leary founded the League for Spiritual Discovery, a religious organization that had LSD as a sacrament. In 1970, Leary was convicted to a ten-year prison sentence for possession of cannabis, and the League shut down in his absence. In 2006, a new League started up, ten years after Leary's death.
After its short popularity during the 1960s, many nations, including the US, made LSD illegal.
In the US, LSD is a Schedule I drug, meaning that the government has determined that it has "a high potential for abuse and serve[s] no legitimate medical purpose."  It is illegal to manufacture, buy, sell, trade, give away, or possess. 
LSD can cause effects such as mild hallucinations, sleeplessness, "enhanced" senses (brighter colors, richer sounds, etc.), increased heartrate, appetite loss, and synaesthesia. Synaesthesia is a phenomenon where senses overlap (for example, tasting colors, or seeing sounds). These effects, colloquially known as a "trip", can last from 8-12 hours.
Adverse mental reactions (a "bad trip") may result from negative emotions (or other environmental factors), or an unusually high dosage. These can be manifested as a sense of grim foreboding, anxiety, or even abject panic. To avoid a trip turning bad, it is suggested to create a change of environment (such as a different location, or listening to soothing music), or to remember that the mental effects are just being caused by the drug, and are only temporary.
The "threshold" (smallest dose with perceptible effects) is 20 micrograms. A "common" dose is between 50 and 150 micrograms. The lethal dosage is 12,000 micrograms, roughly 120 times a common dose. Hoffman, in his book LSD: My Problem Child, notes: "The danger of LSD lies not in its toxicity, but rather in the unpredictability of its psychic effects." 
Because of this unpredictability, many users suggest having a "trip sitter" around during the trip, especially if it's the first time. Ideally, the sitter is a known and trusted friend, and knows how to deal with bad trips.
The DSM-IV lists Hallucinogen Persisting Perception Disorder (HPPD) as a syndrome usually linked to LSD use. Victims of HPPD have altered perceptions, similar to those experienced during an LSD trip. Symptoms include seeing flashes of light, patterns in otherwise blank surfaces, and tracers from moving objects. Fortunately, this syndrome is rare.
- LSD and Strychnine
- Chromosomal damage
Death of Diane LinkletterEdit
Although flashbacks are real, they are uncommon. The myth that LSD builds up deposits in the body is not supported by evidence; the chemical is entirely water-soluble, and leaves the body entirely within hours of ingestion.  It is now believed that flashbacks are caused by HPPD. (See Usage section, above.)