Urban Legends about Cannabis
There are many urban legends about cannabis, perhaps due to its popularity, despite its illegality.

Cannabis causes psychosis, schizophrenia, or clinical depression. There is no evidence that cannabis use directly causes these illnesses. Rather, cannabis may trigger latent conditions or be part of a complex coordination of causes. Also, the physiological effects of cannabis as well as the subjective evidence of some users indicate the likelihood of people with depression using cannabis to alleviate its symptoms.

This opinion was widespread in the psychiatry community in Australia until the last 10 years or so. However the use of cannabis in remote Aboriginal communities, where alcohol restrictions are in place, has changed that thinking. Psychiatric illness has been diagnosed in extra-ordinary rates which have not been seen in those communities before access to cannabis was readily available, or other communities where cannabis is not accessible. The use of "bucket bongs" and extremely heavy consumption seems to be the factor that triggers psychiatric reactions among individuals with no known risk factors and no past history.

Using a bong or water pipe removes tars from the smoke
This is not entirely true; while water will remove some of the tars from the smoke, it will also filter out some of the THC, so the user actually has to smoke more to produce an equivalent high. Some people dispute this however, saying that since THC isn't appreciably water-soluble, the "filtering" effect is not all that prominent, implying that it is indeed safer to smoke pot through water than via a cigarette. Due to the lack of scientifically valid studies on this topic, it is presently impossible to classify this myth as entirely true or false.

A joint is equal in carcinogens to 5 cigarettes

The factual accuracy of this section is disputed. Please view blah, blah, blah.

The concentration of carcinogens in cannabis smoke is actually lower than that in tobacco smoke. Cannabis smokers do, however, tend not to use filters and hold the smoke in their lungs longer. Therefore, they absorb more of the carcinogens present, though a 5x increase over tobacco smoke would be unlikely. The real ratio is nearer to a 300% increase. In addition, a large majority of cancers caused by cigarette smoking are related to radioactive polonium-210, radium-226, and radon. These elements come from the fertilizer used by tobacco growers and are unlikely to be found in most marijuana, espeically with the increasing popularity of indoor growing and hydroponics.

http://encyclopedia.tfd.com/Drug+urban+legends

Some psychedelics (namely LSD, psilocybin and cannabis) are extremely non-toxic, making it nearly impossible to physically overdose.

http://encyclopedia.tfd.com/hallucinogenic+drug