The low cost of LSD seems to appeal to high school students, even as they reportedly curtail their use of marijuana and cocaine. LSD, which belongs to a group of compounds known as ergot alkaloids, is a very powerful hallucinogenic, thought to affect the sertonergic nervous system. Oral doses result in many symptoms including dizziness, weakness, drowsiness, euphoria, sensory illusions, and affective changes. Low doses taken repeatedly over time may produce a tolerance to LSD, which means that the user develops a state of increasing resistance to the usual effects of the drug as a result of prior exposure. The lethal dosage of LSD is still unknown. The DEA believes there are only a dozen American LSD manufacturers, centered in the Northwest; San Francisco is the primary distribution point for the U.S. and most of the world. 2 figures