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Adding Cannabis to Tobacco Smoking Worsens Lung Health

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.

Macleod, John et al.·The British journal of general practice : the journal of the Royal College of General Practitioners·2015·Moderate EvidenceCross-Sectional
Respiratory (136)Harm Reduction (776)
RTHC-01005Cross SectionalModerate Evidence2015RETHINKTHC RESEARCH DATABASErethinkthc.com/research

Quick Facts

Study Type
Cross-Sectional
Evidence
Moderate Evidence
Sample
N=12,500

What This Study 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.

Key Numbers

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 They Did This

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.

Why This Research 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 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.

What This Study Doesn't 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.

Questions This Raises

  • ?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?

Trust & Context

Key Stat:
0.3% increase in COPD prevalence per joint-year of cannabis
Evidence Grade:
Population-based study with standardized spirometry, though cross-sectional design and single-practice sample limit conclusions.
Study Age:
Published in 2015. Vaporizing and other non-combustion methods have become more common since.
Original Title:
Cannabis, tobacco smoking, and lung function: a cross-sectional observational study in a general practice population.
Published In:
The British journal of general practice : the journal of the Royal College of General Practitioners, 65(631), e89-95 (2015)
Database ID:
RTHC-01005

Evidence Hierarchy

Meta-Analysis / Systematic Review
Randomized Controlled Trial
Cohort / Case-Control
Cross-Sectional / ObservationalSnapshot without intervening
This study
Case Report / Animal Study

A snapshot of a population at one point in time.

What do these levels mean? →

Frequently Asked 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 More on RethinkTHC

Cite This Study

RTHC-01005·https://rethinkthc.com/research/RTHC-01005

APA

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

MLA

Macleod, John, et al. "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, 2015. https://doi.org/10.3399/bjgp15X683521

RethinkTHC

RethinkTHC Research Database. "Cannabis, tobacco smoking, and lung function: a cross-sectio..." RTHC-01005. Retrieved from https://rethinkthc.com/research/macleod-2015-cannabis-tobacco-smoking-and

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Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.

This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.

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