Smoking cannabis doesn’t cure cancer but it may cause it

As I have written before, there is a lot of controversy about medical uses for marijuana, although it appears to be much more of a political debate than a scientific one. There just isn’t much evidence that supports a hypothesis that marijuana has any significant therapeutic effect on diseases like cancer, neurological disorders, or other diseases. 
Scientists have long suspected that smoking marijuana could be linked to lung cancer, but there has only been weak evidence supporting a causality.  Recently, a 40 year review of  over 49,000 men strongly suggests that smoking cannabis does indeed increase the risk of lung cancer. The study examined 49,321 men between the ages of 18 and 20 who were being enlisted in the Swedish military between 1969 and 1970, examining their health and lifestyle issues, along with their use of marijuana. The researchers reviewed other potential risk factors such as respiratory disease, other types of smoking, and socioeconomic status.
The researchers found the following:

• About 10.5% of the study members had used cannabis.
• About 1.7% were considered heavy users of marijuana (defined as smoking more than 50 times during their lives).
• 189 cases of lung cancer through 2009.
• Heavy users of marijuana had a 212% higher risk for lung cancer compared to those who never used the drug.
• The higher risk stood even after adjusting for tobacco and alcohol use, respiratory conditions, and socioeconomic status (all of which are considered risk factors for lung cancer).

The authors concluded that “our findings do raise concern about the potential long-term lung cancer risk associated with marijuana use in adolescence and young adulthood, a time of pronounced lung development, especially given the possibility that such marijuana smoking may occur during a ‘critical period’ of lung cancer susceptibility to carcinogens in marijuana smoke.”
Although the data appeared to be statistically significant, it wasn’t exactly a huge increase in risk. Tobacco smoking increases risk of lung cancer by 15-30X over nonsmokers. And even a heavy cannabis smoker (at least according to this study) has only smoked it 50 times or more, while 50 cigarettes may be a one or two day amount for a heavy smoker.
However, according to the study authors, many of the same carcinogens found in cigarette smoke are also found in marijuana smoke. Also, the lungs are extraordinarily sensitive to airborne contaminants, whether it is from cigarette or marijuana smoke, industrial particles like asbestos, or general atmospheric pollution.
Also, this type of study may not show conclusive causality, although the association between heavy marijuana smoking and lung cancer is strong. Further evidence in the form of risk vs. amount used, more age groups, and broader populations might provide more evidence for causality.
Smoking cannabis probably does slightly increase risks for lung cancer, and it would multiply any risk from tobacco smoking. If you’re smoking cannabis for pleasure, it has a risk for cancer like other activities like tobacco, alcohol, or other illicit drugs. On the other hand, if you’re smoking cannabis for some unproven health benefits, then given that there are none supported by any reasonable evidence, the risk of cancer may not be worth it. It’s all about weighing the risks.