Friday, June 12, 2009
The pro marijuana crowd is touting a study that failed to show a carcinogenic effect of smoking pot.
The author of that study, Dr. Tashkin, does an excellent job of reviewing the data in this video. The important thing about the video is for non scientists, both pro and anti marijuana, to understand three points:
1. THC, the major psychoactive agent in marijuana is not the same thing as marijuana smoke. There is extensive evidence that THC has minimal if any health risk.
2. Marijuana smoke contains more known carcinogens than does tobacco smoke. Smoking marijuana would be expected to increase risk of cancer,.
3. In a well done study, the UCLA group failed to find the expected risk from marijuana smoke.
What does this NOT mean?
Contrary to how the MJ fanatics are pusing these data, Dr. Tashkin's stuidy does NOT mean that MJ does NOT cause cancer.
Similar studies of tobacco failed to show the expected result there too. The reasons for those failures, now that we have much more data, are assumed to be "confounding variables," that is differences in the smoker and non smoker groups not identified by the researchers.
Similarly the failure to confirm an expected result does not prove that the concern abut cancer and marijuana goes away. We know that MJ smoke will cause cancer and can explain this by its chemical composition.
What DOES this mean?
This is a surprising result since we know that the tars in marijuana smoke should cause cancer. Moreover we know those tars are getting in because biopsies studies of lung in MJ users show changes consistent with the expected effects of these tars.
So either there were confounding variables or some ingredient of the smoke is protective.
Pot smokers should also be aware that tobacco smoke is also highly implicated in diseases other than lung cancer, eg atherosclerosis and cancer outside the lung.
What is a confounding variable?
In animal studies we are able to control almost every possible difference between two groups. Most animal studies are done with inbred mice, meaning we know that genetically both groups are similar. We also monitor the mice for unexpected badges that can effect an experiment .. e.g. changes in dominance, weight gain, etc.
In human studies we have much less control. Some imaginable confounding variables are:
a. the diet of the marijuana users could be very different from that of non users.
b. the folks who use MJ may come from a different genetic stock than those who do not.
c. MJ smokers may congregate in different neighborhood or work in different jobs than non users.
Could this be an accidental result?
All statistical studies depend on the size of the population and can only produce probability. If enough studies are done some will fail. Again, a number of early tobacco studies also failed to show the risk of tobacco.
Could MJ be anticancer?
The study raises that question. Most of the evidence, however, suggests that MJ smoke is carcinogenic. An anticancer effect would imply that some ingredient of the smoke has an unexpected activity.
Is it wise to smoke MJ?
The answer is much as it was thirty years ago for tobacco. We knew the stuff was loaded with carcinogens. This was no direct evidence for cancer and some early studies pointed the other way.
Anti tobacco folks suggested a a thought experiment long before we had enough human data to make our point. Imagine collecting the tars from an putting then under a bandage on your arm. Based on what we knew about tobacco and now know about marijuana, BOTH should cause a skin cancer. If you are comfortable with the risk, then smoke away .. just do not do it near others who may not want to be in your experiment.
Posted by SM Schwartz at 9:15 AM