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.