The best evidence on lung cancer comes from Dr. Donald Tashkin of UCLA, who is the leading researcher on that topic. His latest research shows that marijuana smokers actually have a slightly lower incidence of lung cancer than people who do not smoke at all. This may be because the active ingredient in marijuana, THC, is known to kill various types of tumors. Marijuana may have a slight protective effect against certain types of cancers.
The most comprehensive long-term study of the effects of marijuana was done by Kaiser Permanente. They studied the records of over 65,000 patients over a period of years. They found that they were no significant differences in the health histories of marijuana smokers versus non-smokers. See http://www.druglibrary.org/crl/aging/sidney-01.html - Source: Cliff Schaffer of the Schaffer Online Library of Drug Policy
If I seem to you like a pseudo drug advocate, you are probably right.