Researchers aren't 100 percent sure what LSD does in the central nervous system, or exactly how it causes those hallucinogenic effects. This is in part because there have never been scientific research studies on how LSD affects the brain. It's believed that LSD works similarly to serotonin, a neurotransmitter responsible for regulating moods, appetite, muscle control, sexuality, sleep and sensory perception. LSD seems to alter the way the brain's serotonin's receptors work. It may inhibit neurotransmission, stimulate it, or both. It also affects the way that the retinas process information and conduct that information to the brain.
As little as 0.25 micrograms of LSD per 2.2 pounds (about 1 kilogram) of body weight causes trips, and that's a fairly standard modern dose. In the 1960s, users commonly ingested four times as much. When a person takes LSD, it's quickly metabolized in the <a>liver and eventually excreted in the urine. A small amount is left in the body by the end of the trip and is probably gone entirely a few weeks afterward.
A common urban legend maintains that LSD stays in the body forever, in minuscule amounts in the brain or spinal fluid. People who believe this say that the brain stores and releases molecules of LSD over time, and this is what causes flashbacks. A flashback occurs when a person who has used LSD in the past has an experience, lasting anywhere from seconds to hours, similar to that of an actual trip. Some LSD users enjoy them and consider them "free trips," while others find them incredibly unsettling.
Most users don't experience flashbacks, and some people claim that they don't really exist, making the subject a controversial one. A study found no link between using psychedelics and experiencing flashbacks. Nevertheless, some psychiatrists say some of their patients report this experience [source: Ferro].
Among people who have reported experiencing flashbacks, many are also mentally ill. Some doctors suggest that what the user perceives as a flashback is really a form of psychosis or mental illness that may have emerged due to LSD use. There's a medically recognized disorder called Hallucinogen Persisting Perceptive Disorder (HPPD), in which some people who have taken a lot of LSD experience persistent visual hallucinations (as opposed to the brief flashbacks). It's not yet known exactly what makes some people more susceptible to these experiences than others.
On the next page, we'll look at worst-case scenarios.