I like to make my position clear from the outset: I support the legalization of cannabis (marijuana and hashish). I think criminalizing it makes criminals out of otherwise upstanding people who use it recreationally, either harmlessly or harmfully (of more, anon). And criminalizing it is a latter-day prohibition, often putting its sale in the hands of organized (and, frankly, disorganized) crime. The fact that cannabis has been illegal has never stopped anyone in Canada from using it and has probably stopped users from admitting their use to their doctor, social worker, parents, etc.

But does it have legitimate medical benefits? Should it be used to help with symptom control in pain, chronic neurodegenerative diseases and so on? Should it be prescribed and monitored, rather than just leaving patients to buy it at Shoppers Drug Mart and experiment with the dosage?

This spring, I attended a panel discussion by cannabis producers as part of the Science Writers and Communicators of Canada conference in Vancouver. Naturally, the producers were in favour of the medical use of cannabis, as this is a potential market for them. It also gives the leaf a patina of credibility and respectability.

I am all in favour of treating cannabis as a potential drug, including research into its safety and effectiveness and licensing for proven uses.

But I heard a few things said that were untrue, skirting serious issues in the medical use of cannabis. Sometimes I wondered whether the speakers had been smoking something.

I want to review a few things that were said.

• Only 3% of cannabis users become dependent.
• Cannabis doesn’t cause lung cancer, or does so at lower rates than tobacco.

Let’s look at these claims. The best study I have found on rates of cannabis dependence is a 2012 review of data to date [Degenhardt and Hall (2012). Extent of illicit drug use and dependence, and their contribution to the global burden of disease. The Lancet 379(9810): 55-70].

It found a 9% rate of dependence among users.

A later study by the same authors estimated the rate of cannabis dependence in Canada at 0.6% of the high-income population in 2010 [Degenhardt et al. (2013). The global epidemiology and contribution of cannabis use and dependence to the global burden of disease: results from the GBD 2010 study. PLoS One 8(10): e76635, Table S4]. The rate is undoubtedly higher for the low-income population, as the authors’ 2012 study found.

This is in line with the recent Statistics Canada survey that found that 14% of Canadians over 15 had used cannabis recently (previous three months), with 56% of that 14% saying they used cannabis daily or weekly.

I have seen people become dependent on cannabis, and my observations are in line with the higher estimates.

So, definitely more than 3% of users.

But whether it causes dependence shouldn’t matter anyway, because, evidently, smoking cannabis doesn’t cause lung cancer. Except that it certainly does.

In fact, it increases the overall risk by five times, according to a case–control study carried out in New Zealand and published in 2008 (Eur Respir J. 2008 Feb; 31(2): 280–286. doi: 10.1183/09031936.00065707. For each year of cannabis smoking, the risk increased 8%.

For every study, there is another that contradicts it. Some other studies have not found this association. One of the problems is that a lot of people who smoke cannabis also smoke tobacco, or smoke both together. The New Zealand study used statistical analysis to separate the effects of cannabis smoking from tobacco smoking, but some other studies have not found a separate effect.

An analysis that combined data from several studies had mixed conclusions [Int J Cancer. 2015 Feb 15; 136(4): 894–903]. On one hand, the authors found that having ever smoked cannabis did not alter the overall risk of lung cancer. On the other hand, risk was clearly higher when they looked at a particular kind of lung cancer – adenocarcinoma – especially if the patient smoked a joint or more per day, and especially if they had smoked for more than 10 years. The difference also showed up in people who had smoked cannabis for 20 years or more, for all types of lung cancer.

OK, the risk of lung cancer associated with cannabis smoking appears to be lower than the 23-fold higher risk linked to tobacco smoking (US estimate in men only). But yes, you could get lung cancer.

There is a growing body of evidence on medical use of cannabis, and more trials are needed. I’ll try to get to this in future blog posts.

But arguments for medical use are not helped by unsupported statements and claims. Risks of dependence and lung disease need to be addressed in medical use. Downplaying them or wishing them away doesn’t help anybody.