I cannot believe nobody has figured this out yet, but cannabis DOES NOT impair brain function, and it DOES NOT cause respiratory diseases (that includes cancer).
All the proof you need is in this video: http://www.youtube.com/watch?v=JBRSsH-ntn0
According to NORML.org:
Marijuana is far less dangerous than alcohol or tobacco. Around 50,000 people die each year from alcohol poisoning. Similarly, more than 400,000 deaths each year are attributed to tobacco smoking. By comparison, marijuana is nontoxic and cannot cause death by overdose. According to the prestigious European medical journal, The Lancet, "The smoking of cannabis, even long-term, is not harmful to health. ... It would be reasonable to judge cannabis as less of a threat ... than alcohol or tobacco."