There is an often circulated myth that once you have taken LSD, it remains in your body forever. One thing that keeps these rumors circulating the is fact that some people (though very few) experience "flashbacks" (generally within a few months after a hallucinogenic experience). It is universally accepted, however, that these flashbacks are not the result of LSD remaining in the system.

LSD is almost entirely metabolized within a day after ingestion. Since the half-life of LSD is only a few hours, only a very small amount of LSD remains even at the end of the trip, and this is excreted in the urine. All traces are undetectable after several days and are certainly gone entirely within a couple of weeks. As detection technology improves and thresholds drop (it is now possible to detect picograms reliably), the time that incredibly small amounts of it could be detected will extend.

It has long been reported that LSD is fully metabolized almost immediately after ingestion. This was based on research done in the 50's and 60s which used instruments not sensitive enough to detect the extremely small amounts of the chemical.

Occasionally, people claim that LSD has been found in spinal fluid years after the last time LSD was taken. There is no support for this claim. If anyone knows of a research article that has looked at spinal fluid of LSD users for LSD, please let us know.

Although research in the 1960s found the half-life of LSD was around 3 hours, more recent research shows that LSD's metabolism takes several hours and its peak plasma levels occur at around 3-5 hours after ingestion. It is important to note that such research is highly dependent on the individuals who were used for the research, with metabolism of many drugs varying by up to 2-3 times, larger numbers of research subjects reduces the likelihood of having a skewed range.

From "Notes on the Persistence of LSD in Humans" by Jim Ketchum MD, working with Aghajanian:

Half-life found to be around 160 minutes mean across 40 subjects. "It seems logical to conclude that in man also, its duration in the brain is finite -- almost certainly less than 24 hours."

From: "Measurement of lysergic acid diethylamide (LSD) in human plasma by gas chromatography/negative ion chemical ionization mass spectrometry." Papac DL, Folts RL , J.Anal. Toxicol., 14, 189-190 (1990)

In a single volunteer given about 70 ug, the apparent plasma half-life of LSD is 5.1 hr. The peak plasma concentration of 1.9 ng/mL occurred 3 hr after administration.

From: "The determination of LSD in human plasma following oral administration", Upshall DG, Wailling DG. Clinc. chim. Acta, 1972; 36:67-73.

LSD was detectable for up to five hours...

From: "A urine screening test of lysergide" Faed EM, McLeod WR. Journal of Chromatographic Science. 11, 4-6 (1973)

"LSD and its metabolites were still detectable in human urine for as long as 4 days after the ingestion of 0.2 mg of the drug."

From LSD My Problem Child, by Albert Hofmann, 1969

The concentration of LSD in the various organs attains maximum values 10 to 15 minutes after injection, then falls off swiftly. The small intestine, in which the concentration attains the maximum within two hours, constitutes an exception. The elimination of LSD is conducted for the most part (up to 80%) through the instestine via liver and bile. Only 1 to 10 percent of the elimination product exists as unaltered LSD; the remainder is made up of various transformation products.

From Psychedelics Encyclopedia :

LSD is a very curious chemical. When given by injection, it disappears rapidly from the blood. It can be observed when tagged with Carbon 14 in all the tissues, particularly the liver, spleen, kidneys, and adrenal glands. The concentration found in the brain is lower than in any other organ -- being only about 0.01 percent of the administered dose. [...]

LSD is highly active when administered orally, absorbed through mucous membranes or through the skin, and is almost completely absorbed by the gastrointestinal tract. Concentrations in the organs reach peak values after only ten to fifteen minutes; then they decrease very rapidly. [...] Some 80 percent of injested LSD is excreted via the liver, bile system and intestinal tract, with only about 8 percent appearing in urine. After two hours, only 1 to 10 percent is still present in the form of unchanged LSD; the rest consists of water soluble metabolites -- such as 2-oxo-2,3-dihydro-LSD -- which do not possess any LSD-type influence on the central nervous system.

Psychic effects of LSD reach their peak about one to three hours following ingestion, when much of the substance has disappeared from the body's major organs, including the brain, though measurable amounts persist in the blood and brain for about eight hours.

From the DEA Web Site :

LSD is absorbed easily from the gastrointestinal tract, and rapidly reaches a high concentration in the blood. It is circulated throughout the body and, subsequently, to the brain. LSD is metabolized in the liver and is excreted in the urine in about 24 hours.

From Pharmacotheon :

The drug is almost completely eliminated from the body before the peak effects begin, suggesting that it acts as a sort of catalyst, inducing neurochemical changes which subsequently result in the entheogenic experience. Only about 1-10% of injected LSD is excreted unaltered, the remainder as a variety of degradation products.

From Psychedelic Drugs Reconsidered :

The half-life of LSD in blood plasma is about two hours.