A minority of alcohol abusers develop severe cerebral dysfunction in the form of Wernicke-Korsakoff syndrome. There is also evidence to suggest that cerebral dysfunction, particularly impaired abstracting ability, occurs in that larger population of heavy drinkers who do not go on to develop the Wernicke-Korsakoff syndrome. There is no consistent evidence that long-term marijuana, hallucinogen, or sedative use causes lasting neuropsychological disturbance. The deficits in abstract thinking reported by some LSD studies are similar to deficits others have reported among alcoholics. Since the LSD studies were not controlled for alcohol use, their interpretation is difficult. It appears that cerebrovascular accidents occur more frequently and at a younger age among amphetamine abusers. There is no reliable information about possible other long-term effects of stimulants on the brain per se (i.e., nonvascular complications). Abuse of intravenous narcotics has been associated with case reports of transverse myelitis and encephalitis. It is not known whether this pathology is a direct or hypersensitivity effect of narcotic drugs, of adulterants, or of infection.