LSD (d-lysergic acid diethylamide) is one of the most potent psychoactive chemicals, this is demonstrated by the fact that unlike other drugs, LSD doses are measured in micrograms, instead of milligrams. Chemist Albert Hofmann, working at the Sandoz Corporation pharmaceutical laboratory in Switzerland, first synthesized LSD in 1938. He was conducting research on possible medical applications of various lysergic acid compounds derived from ergot, a fungus that develops on rye grass, specifically, applications as a respiratory and circulatory stimulant. Searching for compounds with therapeutic value, Hofmann created more than two dozen ergot-derived synthetic molecules. 
LSD is illegally sold in tablets, capsules, and occasionally in liquid form. It is an odorless and colorless substance with a slightly bitter taste that is usually ingested orally. It is often added to absorbent paper, such as blotter paper, and divided into small decorated squares, with each square representing one dose. 
LSD is a Schedule I substance under the Controlled Substances Act. Schedule I drugs, which include Heroin and MDMA, have a high potential for abuse and serve no legitimate medical purpose.  Its two precursors lysergic acid and lysergic acid amide are both in Schedule III of the CSA. The LSD precursors ergotamine and ergonovine are List I chemicals. 
How Does LSD Affect the Brain?
There have been no properly controlled research studies on the specific effects of LSD on the human brain, but smaller studies and several case reports have been published documenting some of the effects associated with the use of hallucinogens:
Sensations and feelings change much more dramatically than the physical signs. The user may feel several different emotions at once or swing rapidly from one emotion to another. If taken in large enough doses, the drug produces delusions and visual hallucinations. The user’s sense of time and self is altered. Experiences may seem to “cross over” different senses, giving the user the feeling of hearing colors and seeing sounds. These changes can be frightening and can cause panic. Some LSD users experience severe, terrifying thoughts and feelings of despair, fear of losing control, or fear of insanity and death while using LSD.
Hallucinogens In General
Hallucinogenic compounds in the form of, or extracted from, plants and mushrooms have been used for centuries, mostly in religious rituals. Almost all hallucinogens contain nitrogen and are classified as alkaloids. Many hallucinogens have chemical structures similar to those of neurotransmitters (e.g., acetylcholine-, serotonin-, or catecholamine-like) and temporarily interfere with their action or bind their receptor sites, but the exact mechanisms by which these substances exert their hallucinogenic effects remain unclear.
The very same characteristics that led to the incorporation of hallucinogens into ritualistic or spiritual traditions have also led to their propagation as drugs of abuse. Importantly, and unlike most other drugs, the effects of hallucinogens are highly variable and characteristically unreliable, producing different effects in different people or at different times. This is mainly due to the significant variations in amount and composition of active compounds, particularly in the hallucinogens derived from plants and mushrooms. Because of their unpredictable nature, the use of hallucinogens can be particularly dangerous.