Does Marijuana Cause Cancer - or Prevent It?

Last week's Healthy Kids post about a new study that regular marijuana use by teens dumbs down their IQs later in life raised a ruckus. Our expert panelist, adolescent specialist Rima Himelstein, MD, cited research finding that marijuana use can increase cancer risk - a connection many commenters questioned loudly.

One commenter pointed out that some studies show compounds in marijuana may have anti-cancer properties. Another contended there's no scientific proof  that marijuana raises risk. Where's the truth? Here's what the research says:

1. Marijuana use is associated with higher risk for testicular cancer. In a 2011 study published in the journal Cancer,  researchers from the National Cancer Institute found that men with testicular cancer were 2.2 times more likely to be regular marijuana smokers (daily or more often) then men without this rare cancer.  A 2009 study from the Fred Hutchinson Cancer Center in Seattle  found the same thing.  Researchers aren't sure exactly why, but note that "The exact mechanism of how heavy marijuana use might increase the risk of TGCT (testicular germ cell tumors)  is unknown, however chronic marijuana exposure has multiple adverse effects on the endocrine and reproductive systems such as gynecomastia, impotence, reduced sperm counts, and suppressed testosterone."

2. Marijuana more than doubles lung cancer risk. French researchers concluded in a 2008 study that marijuana users were 2.4 times more likely to develop lung cancer than non-users. And cancer investigators in New Zealand found that the heaviest marijuana users in their 2008 study had a five-fold higher risk for lung cancer compared to non-users. In general, they found that smoking one joint daily for a year raised risk slightly more than a pack-a-day smoking habit did.

3. Yes, some marijuana compounds seem to protect against cancer -- but others ding DNA and dampen immunity. Cannabinoids extracted from marijuana pushed brain-cancer cells to die faster in one 2009 Spanish study.  And a  2009 Brown University study  found lower odds for squamous-cell cancers of the head and neck among marijuana smokers.

But that's not a great reason to light up. In 2007, Canadian researchers found levels of ammonia, hydrogen cyanide, nitric oxide and other nasty chemicals in marijuana that were three to 20 times higher than in tobacco. The researchers note that while tobacco's toxins -- including 50 that are cancer-causing --  have been studied extensively, those in marijuana haven't.  There's plenty we don't yet know.

But more is being revealed.  In 2009, another team of Canadian scientists watched what happened when they added condensed marijuana or tobacco smoke to animal cells in test tubes.  Marijuana damaged DNA more than tobacco did.  And in 2010, University of South Carolina scientists found that cannabis suppresses immunity - which could leave users' bodies under-equipped to fight cancer.