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Paul Armentano, Deputy Director of the National Organization for the Reform of Marijuana Laws (NORML) and the NORML Foundation, wrote in his May 5, 2006 report "Cannabis Smoke and Cancer: Assessing the Risk":

“Cannabis smoke contains many of the same carcinogens as tobacco smoke, including greater concentrations of certain aromatic hydrocarbons such as benzopyrene, prompting fears that chronic marijuana inhalation may be a risk factor for tobacco-use related cancers.

However, marijuana smoke also contains cannabinoids such as THC (delta-9-tetrahydrocannabinol) and CBD (cannabidiol), which are non-carcinogenic and demonstrate anti-cancer properties in vivo and in vitro…

Cannabis smoke — unlike tobacco smoke — has not been definitely linked to cancer in humans, including those cancers associated with tobacco use. However, certain cellular abnormalities in the lungs have been identified more frequently in long-term smokers of cannabis compared to non-smokers.

Chronic exposure to cannabis smoke has also been associated with the development of pre-cancerous changes in bronchial and epithelium cells in similar rates to tobacco smokers. Cellular abnormalities were most present in individuals who smoked both tobacco and marijuana, implying that cannabis and tobacco smoke may have an additive adverse effect on airway tissue…

However, to date, no epidemiologic studies of cannabis-only smokers have yet to reveal such a finding. Larger, better-controlled studies are warranted.”

May 5, 2006 - Cannabis Smoke and Cancer