Can Smoking Cannabis Cause Cancer?

We often hear about how cannabis might be able to cure cancer. What people ask less is whether cannabis can cause cancer. Well, smoking cannabis is 20 times more likely to cause cancer than tobacco! At least that’s according to the British Lung Foundation’s (BLF) 2012 special report on cannabis…

A conflicted report on conflicting evidence

This claim was picked up wholesale by many news outlets, including the Telegraph, the BBC, and even the NHS but was also heavily critiqued, including by Prof. David Nutt, who called it “scaremongering”^{1 2 3 4}. According to Nutt’s analysis, the BLF report concluded that there was strong evidence that cannabis contributes to lung cancer, based on three key studies. However, in the body of text they admitted that the evidence in humans is “conflicting” and based on “relatively small numbers of people”. Studies also often don’t take into account factors such as the quantity of cannabis smoked or the effects of mixing tobacco with cannabis. It even adds that “some previous evidence suggests that THC may have anti-carcinogenic effects”... The claim is extrapolated from a study by Aldington et al.⁵, while the other two studies, which find either a much smaller increase in risk (2.4 times) or none at all (in the study over the longest period, which would arguable make it the most valid), are largely ignored. Christopher Snowdon examined the claim in depth.

Cannabis and tobacco: A complicated relationship

The problem with joints or spliffs (or even marijuana cigarettes as some like to call them) is that they often contain tobacco. There is no “best tobacco” for spliffs, mixing in any amount or type of tobacco carries the harms of smoking, which is definitely known to increase the risk of lung cancer. Mixing in tobacco also increases the risk of chronic obstructive pulmonary disease (COPD), such as emphysema, as well as that of nicotine addiction and coronary heart disease⁶. The prominence of mixing in tobacco is a real head-scratcher for researchers because it makes everything murkier, making it harder to give definite answers. Still, most studies find no effect of cannabis after adjusting for tobacco.

The BLF report notes that cannabis smoke components are similar to tobacco except for THC (and other cannabinoids...) and nicotine, meaning cannabis smoke has the same carcinogens, but they may be up to 50% higher⁷. Joints tend to be unfiltered and smoked all the way till the end, which is where tar is concentrated, and tar contains high levels of harmful smoke components. Plus, the characteristic deep inhalations and longer intakes mean that the body may retain more of the harmful compounds than when smoking tobacco⁸.

What is often forgotten in scientific studies is that tobacco and cannabis consumption habits are very different. Most users don’t smoke “a pack” of joints a day for several decades, which is when cigarette smoking becomes a serious risk. Even having a joint a day for a decade only equates to 6 months of a pack a day cigarette smoking habit. When looking at tobacco, most studies would consider these individuals a "never smoker". That being said, Hashibe et al. found that even smoking 20,000 joints didn’t significantly increase the risk of lung cancer⁹.

Smoking cannabis does hurt your lungs

Overall, there is a lot of evidence to suggest that cannabis may cause cancer, but when looking in humans, the evidence just isn’t there as of yet. Since the BLF report debacle, studies have continued to struggle to find an association between (lung) cancer and cannabis^{10 11 12}. The BLF report and associated press release seem to have been quietly scrubbed from the internet. The report did still have some interesting findings: such as that ~⅓ of people believed smoking cannabis is harmless. This rises to 40% of under 35s, the age group most likely to have smoked it. Cannabis may not cause cancer but it does cause a whole load of other problems. Check out the risks section of our cannabis guide to find out more about the other health risks it poses and learn about harm reduction to mitigate them.

Cannabis certainly has some harmful effects on the lungs; cannabis smoke has been shown to injure the cell linings of the airways which may explain the increased risk of chronic bronchitis¹³. There is also some evidence that smoking cannabis with tobacco causes a higher risk of COPD than tobacco alone. Smoking cannabis also carries a risk of dependence, if you or one of your friends are struggling with cannabis use, seek help.

How to smoke weed (to reduce lung harm)

We’ve compiled a list of handy tips below to help reduce the harm of cannabis to your lungs. You can find these and more in the harm reduction section of our cannabis guide. Our number one tip is to smoke less cannabis. The best way to avoid harm is simply to not use cannabis. Symptoms like chronic bronchitis will disappear soon after stopping smoking. If you do use cannabis, try to keep for special occasions and avoid making it a habit.

• Stop using tobacco. You can use herbal alternatives or switch to a method that doesn’t require it, such as vaping or inhalation.
• Stop keeping it in. A total myth, gas exchange in your lungs is so quick that keeping it in longer only gives you a head rush from lack of oxygen.
• Use filter tips. Another myth. It won’t “catch all the good stuff” and stop you from getting high.
• Don’t inhale too deeply. This is especially true for bongs and shishas, and it also avoids you getting way too high.
• Vape. This isn’t necessarily harmless (it’s still too early to tell) but it avoids all the harmful chemicals associated with combustion smoke.
• Try edibles. Again, not harmless; edibles take a while to kick in and can be hard to dose meaning it’s easier to take too much and have a bad time but they completely avoid the lungs, which is a definite bonus.

If you plan on using cannabis check out our cannabis harm reduction guide to learn about the experience, dosage, legality and more. All drugs pose risks, and the best way to avoid them is not to take any, but we understand that individuals still choose to so Drugs and Me exists to provide information about drugs to help reduce their harms based on the best evidence available. Drugs and Me is powered by the Alcohol and other Drugs Education Consultancy, to be objective, unbiased, and freely accessible.

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Stay safe!

References:

¹ Adams S. Cannabis health dangers 'underestimated' [Internet]. Telegraph.co.uk. 2012 [cited 20 August 2019]. Available from: https://www.telegraph.co.uk/news/health/news/9311783/drugs-cannabis-lung-cancer-BLF.html

² 'Poor awareness' of cannabis risk [Internet]. BBC News. 2012 [cited 20 August 2019]. Available from: http://www.bbc.co.uk/news/health-18283689

³ Cannabis lung health risks underestimated [Internet]. nhs.uk. 2012 [cited 21 August 2019]. Available from: https://www.nhs.uk/news/heart-and-lungs/cannabis-lung-health-risks-underestimated/

⁴ Nutt D. Smoke without fire? Scaremongering by the British Lung Foundation over cannabis vs tobacco [Internet]. David Nutt's Blog: Evidence not Exaggeration. 2012 [cited 21 August 2019]. Available from: https://profdavidnutt.wordpress.com/2012/06/11/smoke-without-fire-scaremongering-by-the-british-lung-foundation-over-cannabis-vs-tobacco/

⁵ Aldington S, Harwood M, Cox B, Weatherall M, Beckert L, Hansell A et al. Cannabis use and risk of lung cancer: a case-control study. European Respiratory Journal. 2008;31(2):280-286.

⁶ Why is smoking bad for me? [Internet]. British Lung Foundation. [cited 21 August 2019]. Available from: https://www.blf.org.uk/support-for-you/smoking/why-is-smoking-bad-for-me

⁷ Tashkin D. Effects of Marijuana Smoking on the Lung. Annals of the American Thoracic Society. 2013;10(3):239-247.

⁸ WU T, Tashkin D, Djahed B, Rose J. Pulmonary Hazards of Smoking Marijuana as Compared with Tobacco. New England Journal of Medicine. 1988;318(6):347-351.

⁹ Hashibe M, Morgenstern H, Cui Y, Tashkin D, Zhang Z, Cozen W et al. Marijuana Use and the Risk of Lung and Upper Aerodigestive Tract Cancers: Results of a Population-Based Case-Control Study. Cancer Epidemiology Biomarkers & Prevention. 2006;15(10):1829-1834.

¹⁰ Zhang L, Morgenstern H, Greenland S, Chang S, Lazarus P, Teare M et al. Cannabis smoking and lung cancer risk: Pooled analysis in the International Lung Cancer Consortium. International Journal of Cancer. 2014;136(4):894-903.

¹¹ Ribeiro L, Ind P. Marijuana and the lung: hysteria or cause for concern?. Breathe. 2018;14(3):196-205.

¹² Huang Y, Zhang Z, Tashkin D, Feng B, Straif K, Hashibe M. An Epidemiologic Review of Marijuana and Cancer: An Update. Cancer Epidemiology Biomarkers & Prevention. 2015;24(1):15-31.

¹³ Tashkin D. Effects of Marijuana Smoking on the Lung. Annals of the American Thoracic Society. 2013;10(3):239-247.