There is no conclusive evidence that cannabis causes cancer in humans including those cancers associated with tobacco use. However, cellular, genetic and human studies suggest that cannabis smoke may be an important risk factor for the development of respiratory cancer. There is not yet any evidence from controlled studies showing a higher rate of respiratory cancers among chronic cannabis smokers. However, there is evidence of an additive effect of cannabis and tobacco smoking on abnormalities in lung tissue, similar to those that precede lung cancer in tobacco smokers. This means that if you smoke cannabis as well as tobacco it is not healthier than smoking tobacco on its own. These effects are related to the amount of cannabis smoked and it has been argued that cannabis smokers will not smoke as much as tobacco smokers or smoke as long in their lives. Most cannabis smokers have stopped using cannabis by their mid to late twenties. There have been case reports of cancers in the digestive tract of young adults with a history of heavy cannabis use. These findings are significant because these kinds of cancers are rarely found in adults under the age of sixty, even among those who smoke tobacco and drink alcohol. This suggests that cannabis smoking may potentate the effects of other risk factors such as tobacco smoking and is a more important risk factor than tobacco and alcohol use in the early development of respiratory cancers. This is an important warning for smokers of cannabis, but being sensible is always the best precaution.
