Adding Cannabis to Tobacco Smoking Worsens Lung Health | Research

The takeaway

In a study of Scottish smokers, those who smoked both cannabis and tobacco had more respiratory symptoms and a small but significant additional increase in COPD rates compared to tobacco-only smokers.

Read this if you smoke cannabis mixed with tobacco and are concerned about lung health.

0.3% increase in COPD prevalence per joint-year of cannabis

What the researchers found

Researchers recruited 500 people from a Scottish general practice: some smoked only tobacco, others smoked both tobacco and cannabis (predominantly resin mixed with tobacco). Both groups had impaired lung function and respiratory symptoms, but those who also smoked cannabis reported more symptoms.

After adjusting for tobacco exposure, each additional joint-year of cannabis was associated with a 0.3% increase in COPD prevalence. Cannabis smokers had higher total tobacco exposure when accounting for tobacco smoked in joints.

Cannabis and tobacco smokers were younger on average (37 vs. 45 years) but already showing additional respiratory effects beyond what their tobacco use alone would predict.

Why it matters

Most cannabis respiratory research comes from populations where cannabis is smoked pure. In the UK and Europe, mixing cannabis resin with tobacco is the dominant method, and this study specifically examined that pattern.

The numbers in context

500 participants; mean age 45 (tobacco only) vs. 37 (cannabis + tobacco); 0.3% increase in COPD prevalence per joint-year (95% CI 0.0-0.5); median tobacco exposure 25 pack-years (tobacco only) vs. 22.5 including cannabis tobacco (combined group)

How the study worked

Cross-sectional study in a Scottish general practice with 12,500 patients. 500 participants (242 males) completed respiratory questionnaires (NHANES and MRC) and standardized spirometry. Cannabis exposure measured in joint-years alongside tobacco pack-years.

What this study cannot tell us

Cross-sectional design limits causal inference. Predominantly resin cannabis, which may differ from herbal cannabis in smoke composition. Single general practice population. Self-reported cannabis use may be underestimated.

How to read the evidence

Population-based study with standardized spirometry, though cross-sectional design and single-practice sample limit conclusions.

When this study was published

Published in 2015. Vaporizing and other non-combustion methods have become more common since.

The bigger picture

The common European practice of mixing cannabis with tobacco compounds the respiratory risks of both substances. For harm reduction, separating cannabis from tobacco or using non-smoked methods could reduce cumulative lung damage.

Questions still open

Would smoking herbal cannabis without tobacco show the same additional respiratory effect? How does vaporizing cannabis compare to smoking it with tobacco? Is the additional COPD risk clinically meaningful given the small effect size?

Common questions

Does cannabis itself damage the lungs, or is it just the tobacco?

This study found additional respiratory effects from cannabis beyond what tobacco alone would predict, with each joint-year adding a small increase in COPD prevalence. However, the cannabis was smoked mixed with tobacco, making it difficult to fully separate the effects.

Is smoking cannabis without tobacco safer for the lungs?

This study did not test that comparison. However, removing tobacco eliminates a known major cause of respiratory disease. Whether cannabis smoke alone causes significant long-term lung damage remains debated in the literature.

Read the original research

Cannabis, tobacco smoking, and lung function: a cross-sectional observational study in a general practice population.

The British journal of general practice : the journal of the Royal College of General Practitioners, 65(631), e89-95

View on PubMed ↗Publisher’s paper ↗

Citation

Macleod, John; Robertson, Roy; Copeland, Lorraine; McKenzie, James; Elton, Rob; Reid, Peter. (2015). Cannabis, tobacco smoking, and lung function: a cross-sectional observational study in a general practice population.. The British journal of general practice : the journal of the Royal College of General Practitioners, 65(631), e89-95. https://doi.org/10.3399/bjgp15X683521