1. Marijuana causes brain damage

The most celebrated study that claims to
show brain damage is the rhesus monkey study of Dr. Robert
Heath, done in the late 1970s. This study was reviewed by a
distinguished panel of scientists sponsored by the Institute
of Medicine and the National Academy of Sciences.
Their results were published under the title, Marijuana and Health
in 1982. Heath's work was sharply criticized for its
insufficient sample size (only four monkeys), its failure
to control experimental bias, and the misidentification
of normal monkey brain structure as "damaged". Actual
studies of human populations of marijuana users have shown
no evidence of brain damage. For example, two studies from
1977, published in the Journal of the American Medical
Association (JAMA) showed no evidence of brain
damage in heavy users of marijuana. That same year, the
American Medical Association (AMA) officially came
out in favor of decriminalizing marijuana. That's not the
sort of thing you'd expect if the AMA thought marijuana damaged the brain.