[anon1385]

>That's despite both your instinct and my own that "of course any smoke causes cancer of the lungs". No, cannabis smoke has not been demonstrated to do that.

http://erj.ersjournals.com/content/31/2/280.full

The risk of lung cancer increased 8% (95% confidence interval (CI) 2–15) for each joint-yr of cannabis smoking, after adjustment for confounding variables including cigarette smoking, and 7% (95% CI 5–9) for each pack-yr of cigarette smoking, after adjustment for confounding variables including cannabis smoking. The highest tertile of cannabis use was associated with an increased risk of lung cancer (relative risk 5.7 (95% CI 1.5–21.6)), after adjustment for confounding variables including cigarette smoking.

In conclusion, the results of the present study indicate that long-term cannabis use increases the risk of lung cancer in young adults.

[samatman]

What you are showing here is that we've moved on to the meta-analysis part of the scientific method. Basically, given many studies, featuring an unknown number of bad actors and poor methodologies, what conclusion can we draw from the numbers?

There are 400 people enrolled in that study. British health services have conducted multiple longitudinal studies, on Jamaican, British and Indian populations, over more than 100 years, with larger sample sets. Different conclusions were drawn.

Note: what I wrote is not a meta-analysis either. I don't have one in my back pocket. If you find one let's talk.

[socillion]

"Although marijuana smoke contains a number of carcinogens and cocarcinogens, findings from a limited number of well-designed epidemiological studies do not suggest an increased risk for the development of either lung or upper airway cancer from light or moderate use, although evidence is mixed concerning possible carcinogenic risks of heavy, long-term use... Several case reports have implicated marijuana smoking as an etiologic factor in pneumothorax/pneumomediastinum and bullous lung disease, although evidence of a possible causal link from epidemiologic studies is lacking. In summary, the accumulated weight of evidence implies far lower risks for pulmonary complications of even regular heavy use of marijuana compared with the grave pulmonary consequences of tobacco."

https://www.ncbi.nlm.nih.gov/pubmed/23802821

Wildly off topic, though.