DrugPolicy.org has a list called "Myths and Facts About Marijuana"
Myth: Marijuana Can Cause Permanent Mental Illness. Among adolescents, even occasional marijuana use may cause psychological damage. During intoxication, marijuana users become irrational and often behave erratically.
Fact: There is no convincing scientific evidence that marijuana causes psychological damage or mental illness in either teenagers or adults. Some marijuana users experience psychological distress following marijuana ingestion, which may include feelings of panic, anxiety, and paranoia.
Myth: Marijuana is Highly Addictive. Long term marijuana users experience physical dependence and withdrawal, and often need professional drug treatment to break their marijuana habits.
Fact: Most people who smoke marijuana smoke it only occasionally. A small minority of Americans - less than 1 percent - smoke marijuana on a daily basis. An even smaller minority develop a dependence on marijuana. Some people who smoke marijuana heavily and frequently stop without difficulty.
Myth: Marijuana Is More Potent Today Than In The Past. Adults who used marijuana in the 1960s and 1970s fail to realize that when today's youth use marijuana they are using a much more dangerous drug.Fact: When today's youth use marijuana, they are using the same drug used by youth in the 1960s and 1970s. A small number of low-THC samples seized by the Drug Enforcement Administration are used to calculate a dramatic increase in potency. However, these samples were not representative of the marijuana generally available to users during this era. Read More of this article From DrugPolicy.org
DrugText.org has an article called "Marijuana Myths"
This is one of the more persistent myths. A real world example of what happens when marijuana is readily available can be found in Holland. The Dutch partially legalized marijuana in the 1970s. Since then, hard drug use-heroin and cocaine-have DECLINED substantially. If marijuana really were a gateway drug, one would have expected use of hard drugs to have gone up, not down. This apparent "negative gateway" effect has also been observed in the United States. Studies done in the early 1970s showed a negative correlation between use of marijuana and use of alcohol
Marijuana suppresses the immune system
Like the studies claiming to show damage to the reproductive system, this myth is based on studies where animals were given extremely high-in many cases, near-lethal-doses of cannabinoids. These results have never been duplicated in human beings. Interestingly, two studies done in 1978 and one done in 1988 showed that hashish and marijuana may have actually stimulated the immune system in the people studied.
Legal marijuana would cause carnage on the highways
Although marijuana, when used to intoxication, does impair performance in a manner similar to alcohol, actual studies of the effect of marijuana on the automobile accident rate suggest that it poses LESS of a hazard than alcohol. In one test the number of accident victims intoxicated on marijuana relative to the number of marijuana users in society gave a ratio similar to that for accident victims intoxicated on alcohol relative to the total number of alcohol users. However, a closer examination of the victims revealed that around 85% of the people intoxicated on marijuana WERE ALSO INTOXICATED ON ALCOHOL. For people only intoxicated on marijuana, the rate was much lower than for alcohol alone.
Marijuana "flattens" human brainwaves
This is an out-and-out lie perpetrated by the Partnership for a Drug-Free America. A few years ago, they ran a TV ad that purported to show, first, a normal human brainwave, and second, a flat brainwave from a 14-year-old "on marijuana". When researchers called up the TV networks to complain about this commercial, the Partnership had to pull it from the air. It seems that the Partnership faked the flat "marijuana brainwave". In reality, marijuana has the effect of slightly INCREASING alpha wave activity. Alpha waves are associated with meditative and relaxed states which are, in turn, often associated with human creativity. Read More of this article From DrugText.org